Mark functions as static in gfs2/rgrp.c because they are not used
outside this file.
This eliminates the following warning in gfs2/rgrp.c:
fs/gfs2/rgrp.c:1092:5: warning: no previous prototype for ‘gfs2_rgrp_bh_get’ [-Wmissing-prototypes]
fs/gfs2/rgrp.c:1157:5: warning: no previous prototype for ‘update_rgrp_lvb’ [-Wmissing-prototypes]