Btrfs: incremental send, fix emission of invalid clone operations

Btrfs: incremental send, fix emission of invalid clone operations

When doing an incremental send it's possible that the computed send stream
contains clone operations that will fail on the receiver if the receiver
has compression enabled and the clone operations target a sector sized
extent that starts at a zero file offset, is not compressed on the source
filesystem but ends up being compressed and inlined at the destination
filesystem.

Example scenario:

  $ mkfs.btrfs -f /dev/sdb
  $ mount -o compress /dev/sdb /mnt

  # By doing a direct IO write, the data is not compressed.
  $ xfs_io -f -d -c "pwrite -S 0xab 0 4K" /mnt/foobar
  $ btrfs subvolume snapshot -r /mnt /mnt/mysnap1

  $ xfs_io -c "reflink /mnt/foobar 0 8K 4K" /mnt/foobar
  $ btrfs subvolume snapshot -r /mnt /mnt/mysnap2

  $ btrfs send -f /tmp/1.snap /mnt/mysnap1
  $ btrfs send -f /tmp/2.snap -p /mnt/mysnap1 /mnt/mysnap2
  $ umount /mnt

  $ mkfs.btrfs -f /dev/sdc
  $ mount -o compress /dev/sdc /mnt
  $ btrfs receive -f /tmp/1.snap /mnt
  $ btrfs receive -f /tmp/2.snap /mnt
  ERROR: failed to clone extents to foobar
  Operation not supported

The same could be achieved by mounting the source filesystem without
compression and doing a buffered IO write instead of a direct IO one,
and mounting the destination filesystem with compression enabled.

So fix this by issuing regular write operations in the send stream
instead of clone operations when the source offset is zero and the
range has a length matching the sector size.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>