net: qrtr: Remove rcu logic for port lookup/remove

synchronize_rcu() is slow and can create delays on creating/removing
sockets.

Some clients have latency requirements for spinning off a
thread that creates a client and lookup sockets for some sessions. They
close the lookup socket during this sequence which causes delays. Each
call to synchronize_rcu takes 20-40 ms which is too long.

Remove the rcu logic and change qrtr_port_lock to a spinlock so it can
be used in qrtr_port_lookup which may be used in atomic context.

Change-Id: I3244e31d3ca0599df794d3936033735bcdd3e972
Signed-off-by: Chris Lew <clew@codeaurora.org>