Btrfs: fix segmentation fault when doing dio read

Btrfs: fix segmentation fault when doing dio read

[ Upstream commit 97bf5a5589aa3a59c60aa775fc12ec0483fc5002 ]

Commit 2dabb3248453 ("Btrfs: Direct I/O read: Work on sectorsized blocks")
introduced this bug during iterating bio pages in dio read's endio hook,
and it could end up with segment fault of the dio reading task.

So the reason is 'if (nr_sectors--)', and it makes the code assume that
there is one more block in the same page, so page offset is increased and
the bio which is created to repair the bad block then has an incorrect
bvec.bv_offset, and a later access of the page content would throw a
segmentation fault.

This also adds ASSERT to check page offset against page size.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>