gfs2: Trim the ordered write list in gfs2_ordered_write()

We iterate through the entire ordered writes list in
gfs2_ordered_write() to write out inodes. It's a good
place to try and shrink the list by throwing out inodes
that don't have any pages.

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