IB/uverbs: Support for query SRQ from userspace

Add support to uverbs to handle querying userspace SRQs (shared
receive queues), including adding an ABI for marshalling requests and
responses.  The kernel midlayer already has the underlying
ib_query_srq() function.

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