[GFS2] Fix ordering of page disposal vs. glock_dq

In case of unlinked files with dirty pages GFS2 wasn't clearing
the pages in quite the right order. This patch clears the pages
earlier (before the qlock_dq) to avoid the situation that the
release of the glock results in attempting to write back data that
has already been deallocated.

This fixes Red Hat bugzilla: #220117

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