ARM: make user_addr_max more robust

With CONFIG_MMU=y get_fs() returns current_thread_info()->addr_limit
which is initialized as USER_DS (which in turn is defined to TASK_SIZE)
for userspace processes. At least theoretically
current_thread_info()->addr_limit is changable by set_fs() to a
different limit, so checking for KERNEL_DS is more robust.

With !CONFIG_MMU get_fs returns KERNEL_DS. To see what the old variant
did you'd have to find out that USER_DS == KERNEL_DS which isn't needed
any more with the variant this patch introduces. So it's a bit easier to
understand, too.

Also if the limit was changed this limit should be returned, not
TASK_SIZE.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/arm/include/asm/uaccess.h

@@ -224,7 +224,7 @@ static inline void set_fs(mm_segment_t fs)
 #define access_ok(type,addr,size)	(__range_ok(addr,size) == 0)
 
 #define user_addr_max() \
-	(segment_eq(get_fs(), USER_DS) ? TASK_SIZE : ~0UL)
+	(segment_eq(get_fs(), KERNEL_DS) ? ~0UL : get_fs())
 
 /*
  * The "__xxx" versions of the user access functions do not verify the