btrfs: test to exercise shared extent reference accounting

btrfs: test to exercise shared extent reference accounting

Regression test for adding and dropping an equal number of references
for file extents. Verify that if we drop N references for a file extent
and we add too N new references for that same file extent in the same
transaction, running the delayed references (which always happens at
transaction commit time) does not fail.

The regression was introduced in the 4.2-rc1 Linux kernel and fixed by
the patch titled: "Btrfs: fix order by which delayed references are run".

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>