[GFS2] Replace rgrp "recent list" with mru list

This patch removes the "recent list" which is used during allocation
and replaces it with the (already existing) mru list used during
deletion. The "recent list" was not a true mru list leading to a number
of inefficiencies including a "next" function which made scanning the
list an order N^2 operation wrt to the number of list elements.

This should increase allocation performance with large numbers of rgrps.
Its also a useful preparation and cleanup before some further changes
which are planned in this area.

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