s390/xor: Improve inline assembly constraints

s390/xor: Improve inline assembly constraints

The inline assembly constraint for the "bytes" operand is "d" for all xor()
inline assemblies. "d" means that any register from 0 to 15 can be used. If
the compiler would use register 0 then the exrl instruction would not or
the value of "bytes" into the executed instruction - resulting in an
incorrect result.

However all the xor() inline assemblies make hard-coded use of register 0,
and it is correctly listed in the clobber list, so that this cannot happen.

Given that this is quite subtle use the better "a" constraint, which
excludes register 0 from register allocation in any case.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Link: https://lore.kernel.org/r/20260302133500.1560531-3-hca@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>