Fix the resource group wrap-around logic in gfs2_rbm_find that commit
e579ed4f broke.  The bug can lead to unnecessary repeated scanning of the
same bitmaps; there is a risk that future changes will turn this into an
endless loop.

This is an updated version of commit 2d29f6b9 ("gfs2: Fix loop in
gfs2_rbm_find") which ended up being reverted because it introduced a
performance regression in iozone (see commit e74c98ca).  Changes since v1:

 - Simplify the wrap-around logic.

 - Handle the case where each resource group only has a single bitmap block
   (small filesystem).

 - Update rd_extfail_pt whenever we scan the entire bitmap, even when we don't
   start the scan at the very beginning of the bitmap.

Fixes: e579ed4f ("GFS2: Introduce rbm field bii")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>