dm-ima: use SHA-256 library

dm-ima: use SHA-256 library

Make dm_ima_measure_on_table_load() use the SHA-256 library API instead
of crypto_shash to calculate the SHA-256 hash value that it needs.  This
is simpler and more efficient.  It also ensures that SHA-256 is actually
available and doesn't fail due to the unreliable loading by name.

While doing this, also use kasprintf() to simplify building the string
version of the digest.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>