cxgb4: prevent simultaneous execution of service_ofldq()

Change mutual exclusion mechanism to prevent multiple threads of
execution from running in service_ofldq() at the same time.  The old
mechanism used an implicit guard on the down-call path and none on the
restart path and wasn't working. This checking makes the mechanism
explicit and is much easier to understand as a result.

Based on original work by Casey Leedom <leedom@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>