IB/ipath: Allow receive ports mapped into userspace to be shared

Improve port-sharing performance by allowing any process to receive
packets from the shared hardware port under a spin lock for mutual
exclusion. Previously, one process was nominated as the master and
that process was responsible for receiving all packets from the shared
hardware port and either consuming them or forwarding them to their
destination. This led to starvation problems for other processes when
the master process was busy in computation phases.

Signed-off-by: Bryan O'Sullivan <bryan.osullivan@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>