nfs/drc: Fix memory corruptions

* A wrong memory allocator was used to (de)allocate nodes (not data in
them) of rb tree. This patch uses default allocator, since that suits
our purpose.
* Fix reference counting of client, though hitting the codepath
containing this bug is highly unlikely.

Change-Id: I7692097351d6e54288fee01da5af18e761fd0e8c
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
BUG: 1067256
Reviewed-on: http://review.gluster.org/7816
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>