[GFS2] Faster gfs2_bitfit algorithm

This version of the gfs2_bitfit algorithm includes the latest
suggestions from Steve Whitehouse.  It is typically eight to
ten times faster than the version we're using today.  If there
is a lot of metadata mixed in (lots of small files) the
algorithm is often 15 times faster, and given the right
conditions, I've seen peaks of 20 times faster.

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