fsverity: don't use bio_first_page_all() in fsverity_verify_bio()

fsverity: don't use bio_first_page_all() in fsverity_verify_bio()

bio_first_page_all(bio)->mapping->host is not compatible with large
folios, since the first page of the bio is not necessarily the head page
of the folio, and therefore it might not have the mapping pointer set.

Therefore, move the dereference of ->mapping->host into
verify_data_blocks(), which works with a folio.

(Like the commit that this Fixes, this hasn't actually been tested with
large folios yet, since the filesystems that use fs/verity/ don't
support that yet.  But based on code review, I think this is needed.)

Fixes: 5d0f0e57ed90 ("fsverity: support verifying data from large folios")
Link: https://lore.kernel.org/r/20230604022101.48342-1-ebiggers@kernel.org
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>