IB: Set GIDs correctly in ib_create_ah_from_wc()

ib_create_ah_from_wc() doesn't create the correct return address (AH)
when there is a GRH present (source & dest GIDs need to be swapped).

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