fsverity: start consolidating pagecache code

fsverity: start consolidating pagecache code

ext4 and f2fs are largely using the same code to read a page full
of Merkle tree blocks from the page cache, and the upcoming xfs
fsverity support would add another copy.

Move the ext4 code to fs/verity/ and use it in f2fs as well.  For f2fs
this removes the previous f2fs-specific error injection, but otherwise
the behavior remains unchanged.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Link: https://lore.kernel.org/r/20260128152630.627409-7-hch@lst.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>