rv: Unlock on error path in rv_unregister_reactor()

Unlock the "rv_interface_lock" mutex before returning.

Link: https://lkml.kernel.org/r/YuvYzNfGMgV+PIhd@kili



Fixes: 04acadcb ("rv: Add runtime reactors interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>