[GFS2] Fix leak of gfs2_bufdata

This fixes a memory leak of struct gfs2_bufdata and also some
problems in the ordered write handling code. It needs a bit
more testing, but I believe that the reference counting of
ordered write buffers should now be correct.

This is aimed at fixing Red Hat bugzilla: #201028 and #201082

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