btrfs/035: check for data loss

btrfs/035: check for data loss

The test currently verifies that cloning one file with an inline extent
with a size of 10 bytes into a file with an inline extent that has a size
of 20 bytes succeeds. But this results in data loss, because the btrfs
clone operation drops the 20 bytes inline extent from the destination
inode and then copies the 10 bytes inline extent from the source file
into the destination file, resulting in data loss of the last 10 bytes
of data that the destination file had.

Fixing btrfs to correctly operate for this case (not resulting in data
loss) is actually a lot of work and brings a lot of complexity, specially
considering that any of the inline extents can be compressed. For the
moment there's a fix to make the clone operation return the errno
EOPNOTSUPP and not touch any of the inodes. This is the same approach
we do for other cases involving operation against inline extents, so
this just adds one more case that should have never been allowed.
Cloning inline extents is a rare operation and pointless, since it
involves copying them and not doing any actual deduplication or saving
space.

The btrfs patch for the linux kernel that prevents this data loss,
and fixes some file corruption cases, is titled:

  "Btrfs: fix file corruption and data loss after cloning inline extents"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>