ext4: consolidate fsverity_info lookup

ext4: consolidate fsverity_info lookup

Look up the fsverity_info once in ext4_mpage_readpages, and then use it
for the readahead, local verification of holes and pass it along to the
I/O completion workqueue in struct bio_post_read_ctx.

This amortizes the lookup better once it becomes less efficient.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Link: https://lore.kernel.org/r/20260202060754.270269-9-hch@lst.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>