[PATCH] IB/mthca: Don't try to set srq->last for userspace SRQs

Userspace SRQs don't have a buffer allocated for them in the kernel, so
it doesn't make sense to set srq->last during initialization.  In fact,
this can crash trying to follow a nonexistent buffer pointer.

Signed-off-by: Roland Dreier <rolandd@cisco.com>