fs-verity: use mempool for hash requests

When initializing an fs-verity hash algorithm, also initialize a mempool
that contains a single preallocated hash request object.  Then replace
the direct calls to ahash_request_alloc() and ahash_request_free() with
allocating and freeing from this mempool.

This eliminates the possibility of the allocation failing, which is
desirable for the I/O path.

This doesn't cause deadlocks because there's no case where multiple hash
requests are needed at a time to make forward progress.

Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Theodore Ts'o <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>

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