KVM: s390: selftest: memop: Fix undefined behavior

If an integer's type has x bits, shifting the integer left by x or more
is undefined behavior.
This can happen in the rotate function when attempting to do a rotation
of the whole value by 0.

Fixes: 0dd714bf ("KVM: s390: selftest: memop: Add cmpxchg tests")
Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Link: https://lore.kernel.org/r/20240111094805.363047-1-nsg@linux.ibm.comAcked-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20240111094805.363047-1-nsg@linux.ibm.com>

tools/testing/selftests/kvm/s390x/memop.c

@@ -489,6 +489,8 @@ static __uint128_t rotate(int size, __uint128_t val, int amount)
 
 	amount = (amount + bits) % bits;
 	val = cut_to_size(size, val);
+	if (!amount)
+		return val;
 	return (val << (bits - amount)) | (val >> amount);
 }