[GFS2] Fix bz 230143, incorrect flushing of rgrps

The below patch fixes a problem where we were not flushing rgrps
correctly. It only occurred in the specific case that a callback was
received for an rgrp which was dirty and when a journal log flush had
not already resulted in the rgrp being flushed anyway. This fixes Red
Hat bz 230143,

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