fsverity: push out fsverity_info lookup

fsverity: push out fsverity_info lookup

Pass a struct fsverity_info to the verification and readahead helpers,
and push the lookup into the callers.  Right now this is a very dumb
almost mechanic move that open codes a lot of fsverity_info_addr() calls
in the file systems.  The subsequent patches will clean this up.

This prepares for reducing the number of fsverity_info lookups, which
will allow to amortize them better when using a more expensive lookup
method.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Acked-by: David Sterba <dsterba@suse.com> # btrfs
Link: https://lore.kernel.org/r/20260202060754.270269-7-hch@lst.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>