include/linux/bitrev.h: fix constant bitrev

clang points out with hundreds of warnings that the bitrev macros have a
problem with constant input:

  drivers/hwmon/sht15.c:187:11: error: variable '__x' is uninitialized when used within its own initialization
        [-Werror,-Wuninitialized]
          u8 crc = bitrev8(data->val_status & 0x0F);
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  include/linux/bitrev.h:102:21: note: expanded from macro 'bitrev8'
          __constant_bitrev8(__x) :                       \
          ~~~~~~~~~~~~~~~~~~~^~~~
  include/linux/bitrev.h:67:11: note: expanded from macro '__constant_bitrev8'
          u8 __x = x;                     \
             ~~~   ^

Both the bitrev and the __constant_bitrev macros use an internal
variable named __x, which goes horribly wrong when passing one to the
other.

The obvious fix is to rename one of the variables, so this adds an extra
'_'.

It seems we got away with this because

 - there are only a few drivers using bitrev macros

 - usually there are no constant arguments to those

 - when they are constant, they tend to be either 0 or (unsigned)-1
   (drivers/isdn/i4l/isdnhdlc.o, drivers/iio/amplifiers/ad8366.c) and
   give the correct result by pure chance.

In fact, the only driver that I could find that gets different results
with this is drivers/net/wan/slic_ds26522.c, which in turn is a driver
for fairly rare hardware (adding the maintainer to Cc for testing).

Link: http://lkml.kernel.org/r/20190322140503.123580-1-arnd@arndb.de
Fixes: 556d2f05 ("ARM: 8187/1: add CONFIG_HAVE_ARCH_BITREVERSE to support rbit instruction")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Zhao Qiang <qiang.zhao@nxp.com>
Cc: Yalin Wang <yalin.wang@sonymobile.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/bitrev.h

@@ -34,41 +34,41 @@ static inline u32 __bitrev32(u32 x)
 
 #define __constant_bitrev32(x)	\
 ({					\
-	u32 __x = x;			\
-	__x = (__x >> 16) | (__x << 16);	\
-	__x = ((__x & (u32)0xFF00FF00UL) >> 8) | ((__x & (u32)0x00FF00FFUL) << 8);	\
-	__x = ((__x & (u32)0xF0F0F0F0UL) >> 4) | ((__x & (u32)0x0F0F0F0FUL) << 4);	\
-	__x = ((__x & (u32)0xCCCCCCCCUL) >> 2) | ((__x & (u32)0x33333333UL) << 2);	\
-	__x = ((__x & (u32)0xAAAAAAAAUL) >> 1) | ((__x & (u32)0x55555555UL) << 1);	\
-	__x;								\
+	u32 ___x = x;			\
+	___x = (___x >> 16) | (___x << 16);	\
+	___x = ((___x & (u32)0xFF00FF00UL) >> 8) | ((___x & (u32)0x00FF00FFUL) << 8);	\
+	___x = ((___x & (u32)0xF0F0F0F0UL) >> 4) | ((___x & (u32)0x0F0F0F0FUL) << 4);	\
+	___x = ((___x & (u32)0xCCCCCCCCUL) >> 2) | ((___x & (u32)0x33333333UL) << 2);	\
+	___x = ((___x & (u32)0xAAAAAAAAUL) >> 1) | ((___x & (u32)0x55555555UL) << 1);	\
+	___x;								\
 })
 
 #define __constant_bitrev16(x)	\
 ({					\
-	u16 __x = x;			\
-	__x = (__x >> 8) | (__x << 8);	\
-	__x = ((__x & (u16)0xF0F0U) >> 4) | ((__x & (u16)0x0F0FU) << 4);	\
-	__x = ((__x & (u16)0xCCCCU) >> 2) | ((__x & (u16)0x3333U) << 2);	\
-	__x = ((__x & (u16)0xAAAAU) >> 1) | ((__x & (u16)0x5555U) << 1);	\
-	__x;								\
+	u16 ___x = x;			\
+	___x = (___x >> 8) | (___x << 8);	\
+	___x = ((___x & (u16)0xF0F0U) >> 4) | ((___x & (u16)0x0F0FU) << 4);	\
+	___x = ((___x & (u16)0xCCCCU) >> 2) | ((___x & (u16)0x3333U) << 2);	\
+	___x = ((___x & (u16)0xAAAAU) >> 1) | ((___x & (u16)0x5555U) << 1);	\
+	___x;								\
 })
 
 #define __constant_bitrev8x4(x) \
 ({			\
-	u32 __x = x;	\
-	__x = ((__x & (u32)0xF0F0F0F0UL) >> 4) | ((__x & (u32)0x0F0F0F0FUL) << 4);	\
-	__x = ((__x & (u32)0xCCCCCCCCUL) >> 2) | ((__x & (u32)0x33333333UL) << 2);	\
-	__x = ((__x & (u32)0xAAAAAAAAUL) >> 1) | ((__x & (u32)0x55555555UL) << 1);	\
-	__x;								\
+	u32 ___x = x;	\
+	___x = ((___x & (u32)0xF0F0F0F0UL) >> 4) | ((___x & (u32)0x0F0F0F0FUL) << 4);	\
+	___x = ((___x & (u32)0xCCCCCCCCUL) >> 2) | ((___x & (u32)0x33333333UL) << 2);	\
+	___x = ((___x & (u32)0xAAAAAAAAUL) >> 1) | ((___x & (u32)0x55555555UL) << 1);	\
+	___x;								\
 })
 
 #define __constant_bitrev8(x)	\
 ({					\
-	u8 __x = x;			\
-	__x = (__x >> 4) | (__x << 4);	\
-	__x = ((__x & (u8)0xCCU) >> 2) | ((__x & (u8)0x33U) << 2);	\
-	__x = ((__x & (u8)0xAAU) >> 1) | ((__x & (u8)0x55U) << 1);	\
-	__x;								\
+	u8 ___x = x;			\
+	___x = (___x >> 4) | (___x << 4);	\
+	___x = ((___x & (u8)0xCCU) >> 2) | ((___x & (u8)0x33U) << 2);	\
+	___x = ((___x & (u8)0xAAU) >> 1) | ((___x & (u8)0x55U) << 1);	\
+	___x;								\
 })
 
 #define bitrev32(x) \