rpc/rpc-lib: rpcsvc should reply when rpc_err is set

Problem:
When requests are received on a connection before setvolume
is done, creating frame from the requests fail because there is
no association of the transport with the conn(i.e. xl_private).
xl_private is set only on set_volume. In such cases error response
is not sent from server xlator to that request because of which
operations on mount point are hanging.

Fix:
Set actor return value to RPCSVC_ACTOR_ERROR so that response is sent
even in these cases.

Change-Id: I74d7bc6849fde6c734008d67c1f4bc9d9f7a84f9
BUG: 1006367
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/5892
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>