IB/uverbs: Return actual capacity from create SRQ operation

Pass actual capacity of created SRQ back to userspace, so that
userspace can report accurate capacities.  This requires an ABI bump,
to change struct ib_uverbs_create_srq_resp.

Signed-off-by: Dotan Barak <dotanb@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>