fsverity: simplify fsverity_get_digest()

Instead of looking up the algorithm by name in hash_algo_name[] to get
its hash_algo ID, just store the hash_algo ID in the fsverity_hash_alg
struct.  Verify at boot time that every fsverity_hash_alg has a valid
hash_algo ID with matching digest size.

Remove an unnecessary memset() of the whole digest array to 0 before the
digest is copied into it.

Finally, remove the pr_debug statement.  There is already a pr_debug for
the fsverity digest when the file is opened.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/20221129045139.69803-1-ebiggers@kernel.org