[GFS2] Prevent infinite loop in try_rgrp_unlink()

This is patch three of five for bug #248176.

The try_rgrp_unlink code in rgrp.c had an infinite loop.  This was
caused because the bitmap function rgblk_search can return a block
less than the "goal" block, in which case it was looping.  The fix is
to make it always march forward as needed.

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