IB/sa: Export function to pack a path record into wire format

Allow converting from struct ib_sa_path_rec to the IB defined SA path
record wire format.  This will be used to report path data from the
rdma cm into user space.

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