btrfs: fsync after extent cloning

btrfs: fsync after extent cloning

Test that if we fsync a file that got one extent partially cloned into a
lower file offset, after a power failure our file has the same content it
had before the power failure and after the extent cloning operation.

This test is motivated by an issue found in btrfs that is fixed by the
linux kernel patch titled:

  "Btrfs: fix file read corruption after extent cloning and fsync"

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