GFS2: fix sparse warning: Should it be static?

Impact: Make symbol static.

Fix this sparse warning:
  fs/gfs2/rgrp.c:188:5: warning: symbol 'gfs2_bitfit' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>