kexec_file: use SHA-256 library API instead of crypto_shash API

This user of SHA-256 does not support any other algorithm, so the
crypto_shash abstraction provides no value.  Just use the SHA-256 library
API instead, which is much simpler and easier to use.

Tested with '/sbin/kexec --kexec-file-syscall'.

Link: https://lkml.kernel.org/r/20250428185721.844686-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>