MADs sent to the SA should use the the default P_Key (0x7fff/0xffff).
There's no requirement that the default P_Key is stored at index 0 in
the local P_Key table, so add code to the sa_query module to look up
the index of the default P_Key when creating an address handle for the
SA (which is done any time the P_Key table might change), and use this
index for all SA queries.

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