[GFS2] Add list empty test to databuf_lo_add

Heinz had spotted that I'd forgotten to test in databuf_lo_add()
that the data buffer in question hadn't already been added to
the list. This was causing an infinite loop later on in the
"before commit" routine.

This means that GFS2 is now ready to be tested by everybody.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>