fsverity: explicitly check that there is no algorithm 0

Since libfsverity and some other code would break if 0 is ever allocated
as an FS_VERITY_HASH_ALG_* value, make fsverity_check_hash_algs()
explicitly check that there is no algorithm 0.

Link: https://lore.kernel.org/r/20230705211719.37713-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>