gNFS: gNFS drc cache failed to detecte duplicates.

After the drc cache get full, message "DRC failed to detect duplicates" keep
printed in the log.
The root cause is drc_compare_reqs use the wrong compare type. This function
should use type drc_cache_op_t as its input type. Since all rbtree related code
(except function rpcsvc_drc_lookup) in drc cache pass drc_cache_op_t as compare
type. Only rpcsvc_drc_lookup use type rpcsvc_request_t. It has been modified
too.

Change-Id: I925c097debe6b82f267986961fd4e7755f3de9af
BUG: 1089676
Signed-off-by: Yuan Ding <beback198611@gmail.com>
Reviewed-on: http://review.gluster.org/7519
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>