GFS2: Fix case where reservation finished at end of rgrp

One corner case which the original patch failed to take into
account was when there is a reservation which ended such that
the following block was one beyond the end of the rgrp in
question. This extra test fixes that case.

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