crypto: s390/sha - Fix uninitialized variable in SHA-1 and SHA-2

crypto: s390/sha - Fix uninitialized variable in SHA-1 and SHA-2

Commit 88c02b3f79a6 ("s390/sha3: Support sha3 performance enhancements")
added the field s390_sha_ctx::first_message_part and made it be used by
s390_sha_update() (now s390_sha_update_blocks()).  At the time,
s390_sha_update() was used by all the s390 SHA-1, SHA-2, and SHA-3
algorithms.  However, only the initialization functions for SHA-3 were
updated, leaving SHA-1 and SHA-2 using first_message_part uninitialized.

This could cause e.g. the function code CPACF_KIMD_SHA_512 |
CPACF_KIMD_NIP to be used instead of just CPACF_KIMD_SHA_512.  This
apparently was harmless, as the SHA-1 and SHA-2 function codes ignore
CPACF_KIMD_NIP; it is recognized only by the SHA-3 function codes
(https://lore.kernel.org/r/73477fe9-a1dc-4e38-98a6-eba9921e8afa@linux.ibm.com/).
Therefore, this bug was found only when first_message_part was later
converted to a boolean and UBSAN detected its uninitialized use.
Regardless, let's fix this by just initializing to zero.

Note: in 6.16, we need to patch SHA-1, SHA-384, and SHA-512.  In 6.15
and earlier, we'll also need to patch SHA-224 and SHA-256, as they
hadn't yet been librarified (which incidentally fixed this bug).

Fixes: 88c02b3f79a6 ("s390/sha3: Support sha3 performance enhancements")
Cc: stable@vger.kernel.org
Reported-by: Ingo Franzki <ifranzki@linux.ibm.com>
Closes: https://lore.kernel.org/r/12740696-595c-4604-873e-aefe8b405fbf@linux.ibm.com
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20250703172316.7914-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>