IB/ucm: Get rid of duplicate P_Key parameter

The P_Key is provided into a SIDR REQ in two places, once as a
parameter, and again in the path record.  Remove the P_Key as a
parameter and always use the one given in the path record.

This change has no practical effect on ABI functionality.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>