GFS2: Sort the ordered write list

This patch sorts the ordered write list for GFS2 writes.
This increases the throughput for simultaneous writes.
For example, if you have ten processes, all doing:
dd if=/dev/zero of=/mnt/gfs2/fileX
on different files, the throughput will be much better.

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