GFS2: Sort buffer lists by inplace block number

This patch simply sort the data and metadata buffer lists by their
inplace block number.  This makes gfs2_log_flush issue the inplace IO
in sequential order, which will hopefully speed up writing the IO
out to disk.

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