btrfs: add test for btrfs clone + fsync durability

btrfs: add test for btrfs clone + fsync durability

Regression test for btrfs ioctl clone operation + fsync + log
recovery. The issue was that doing an fsync after cloning into
a file didn't gave any persistence guarantees as it should.
What happened was that the in memory metadata (extent maps)
weren't updated, which made the fsync code not able to detect
that file data has been changed and must be persisted to the
log.

This issue is fixed by the following linux kernel btrfs patch:

    Btrfs: make fsync work after cloning into a file

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