s390: fix test_fp_ctl inline assembly contraints

commit bcf4dd5f upstream.

The test_fp_ctl function is used to test if a given value is a valid
floating-point control. The inline assembly in test_fp_ctl uses an
incorrect constraint for the 'orig_fpc' variable. If the compiler
chooses the same register for 'fpc' and 'orig_fpc' the test_fp_ctl()
function always returns true. This allows user space to trigger
kernel oopses with invalid floating-point control values on the
signal stack.

This problem has been introduced with git commit 4725c860
"s390: fix save and restore of the floating-point-control register"
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/s390/include/asm/switch_to.h

@@ -28,7 +28,7 @@ static inline int test_fp_ctl(u32 fpc)
 		"	la	%0,0\n"
 		"1:\n"
 		EX_TABLE(0b,1b)
-		: "=d" (rc), "=d" (orig_fpc)
+		: "=d" (rc), "=&d" (orig_fpc)
 		: "d" (fpc), "0" (-EINVAL));
 	return rc;
 }