IB/cm: Expose BTH P_Key in CM and SIDR request events

The rdma_cm module will later use the P_Key from the BTH to de-mux
requests.

See discussion at:
  http://www.spinics.net/lists/netdev/msg336067.html

Cc: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: Liran Liss <liranl@mellanox.com>
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>