btrfs: scrub: Don't use inode pages for device replace

[BUG]
Btrfs can create compressed extent without checksum (even though it
shouldn't), and if we then try to replace device containing such extent,
the result device will contain all the uncompressed data instead of the
compressed one.

Test case already submitted to fstests:
https://patchwork.kernel.org/patch/10442353/



[CAUSE]
When handling compressed extent without checksum, device replace will
goe into copy_nocow_pages() function.

In that function, btrfs will get all inodes referring to this data
extents and then use find_or_create_page() to get pages direct from that
inode.

The problem here is, pages directly from inode are always uncompressed.
And for compressed data extent, they mismatch with on-disk data.
Thus this leads to corrupted compressed data extent written to replace
device.

[FIX]
In this attempt, we could just remove the "optimization" branch, and let
unified scrub_pages() to handle it.

Although scrub_pages() won't bother reusing page cache, it will be a
little slower, but it does the correct csum checking and won't cause
such data corruption caused by "optimization".

Note about the fix: this is the minimal fix that can be backported to
older stable trees without conflicts. The whole callchain from
copy_nocow_pages() can be deleted, and will be in followup patches.

Fixes: ff023aac ("Btrfs: add code to scrub to copy read data to another disk")
CC: stable@vger.kernel.org # 4.4+
Reported-by: James Harvey <jamespharvey20@gmail.com>
Reviewed-by: James Harvey <jamespharvey20@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
[ remove code removal, add note why ]
Signed-off-by: David Sterba <dsterba@suse.com>