IB/core: Allow setting create flags in QP init attribute

Allow setting IB_QP_CREATE_BLOCK_MULTICAST_LOOPBACK at create_flags in
ib_uverbs_create_qp_ex.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>