IB/ipath: fix spinlock recursion bug

The local loopback path for RC can lock the rkey table lock without
blocking interrupts.  The receive interrupt path can then call
ipath_rkey_ok() and deadlock.  Remove the redundant lock.

Signed-off-by: Bryan O'Sullivan <bos@pathscale.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>