glusterd: Give up biglock before brick's rpc unref

This is to prevent the possibility of a deadlock when
rpc_connection_cleanup being called in the same thread as rpc_clnt_unref

Change-Id: Ia4dcc0a8a6e6158d4ddec68b780fccbc4cd64adb
BUG: 962619
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/5326
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>