fsverity: constify fsverity_hash_alg

Now that fsverity_hash_alg doesn't have an embedded mempool, it can be
'const' almost everywhere.  Add it.

Link: https://lore.kernel.org/r/20230604022348.48658-1-ebiggers@kernel.org


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