IB/mlx4: Add support for XRC SRQs

Allow the user to create XRC SRQs.  This patch is based on a patch
from Jack Morgenstrein <jackm@dev.mellanox.co.il>.

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