IB/srpt: Set srq_type to IB_SRQT_BASIC

Since commit 96104eda ("RDMA/core: Add SRQ type field"), kernel
users of SRQs need to specify srq_type = IB_SRQT_BASIC in struct
ib_srq_init_attr, or else most low-level drivers will fail in
when srpt_add_one() calls ib_create_srq() and gets -ENOSYS.

(mlx4_ib works OK nearly all of the time, because it just needs
srq_type != IB_SRQT_XRC.  And apparently nearly everyone using
ib_srpt is using mlx4 hardware)

Reported-by: Alexey Shvetsov <alexxy@gentoo.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>