IB/qib: Use RCU for qpn lookup

The heavy weight spinlock in qib_lookup_qpn() is replaced with RCU.
The hash list itself is now accessed via jhash functions instead of mod.

The changes should benefit multiple receive contexts in different
processors by not contending for the lock just to read the hash
structures.

The patch also adds a lookaside_qp (pointer) and a lookaside_qpn in
the context.  The interrupt handler will test the current packet's qpn
against lookaside_qpn if the lookaside_qp pointer is non-NULL.  The
pointer is NULL'ed when the interrupt handler exits.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/qib/qib_verbs.h

@@ -724,7 +724,8 @@ struct qib_ibdev {
 	dma_addr_t pio_hdrs_phys;
 	/* list of QPs waiting for RNR timer */
 	spinlock_t pending_lock; /* protect wait lists, PMA counters, etc. */
-	unsigned qp_table_size; /* size of the hash table */
+	u32 qp_table_size; /* size of the hash table */
+	u32 qp_rnd; /* random bytes for hash */
 	spinlock_t qpt_lock;
 
 	u32 n_piowait;