[GFS2] Detach buf data during in-place writeback

This is patch 5 of 5 for bug #248176

Metadata corruption was occurring because page references weren't
being removed in all cases.  I previously added a function called
detach_bufdata, but I discovered there already WAS a function out
there to do the job.  It's called gfs2_meta_cache_flush.  So I added
a call to that to remove the page references.

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