crypto: sha256 - fix crash at kexec

Loading a large (~2.1G) files with kexec crashes the host with when
running:

  # kexec --load kernel --initrd initrd_with_2G_or_more

  UBSAN: signed-integer-overflow in ./include/crypto/sha256_base.h:64:19
  34152083 * 64 cannot be represented in type 'int'
  ...
  BUG: unable to handle page fault for address: ff9fffff83b624c0
  sha256_update (lib/crypto/sha256.c:137)
  crypto_sha256_update (crypto/sha256_generic.c:40)
  kexec_calculate_store_digests (kernel/kexec_file.c:769)
  __se_sys_kexec_file_load (kernel/kexec_file.c:397 kernel/kexec_file.c:332)
  ...

(Line numbers based on commit da274362 ("Linux 6.12.49")

This started happening after commit f4da7afe
("kexec_file: increase maximum file size to 4G") that landed in v6.0,
which increased the file size for kexec.

This is not happening upstream (v6.16+), given that `block` type was
upgraded from "int" to "size_t" in commit 74a43a2c ("crypto:
lib/sha256 - Move partial block handling out")

Upgrade the block type similar to the commit above, avoiding hitting the
overflow.

This patch is only suitable for the stable tree, and before 6.16, which
got commit 74a43a2c ("crypto: lib/sha256 - Move partial block
handling out"). This is not required before f4da7afe ("kexec_file:
increase maximum file size to 4G"). In other words, this fix is required
between versions v6.0 and v6.16.

Signed-off-by: Breno Leitao <leitao@debian.org>
Fixes: f4da7afe ("kexec_file: increase maximum file size to 4G") # Before v6.16
Reported-by: Michael van der Westhuizen <rmikey@meta.com>
Reported-by: Tobias Fleig <tfleig@meta.com>
Reviewed-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>