RDMA/core: Add SRQ type field

Currently, there is only a single ("basic") type of SRQ, but with XRC
support we will add a second.  Prepare for this by defining an SRQ type
and setting all current users to IB_SRQT_BASIC.

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