RDMA/ucma: Don't allow setting RDMA_OPTION_IB_PATH without an RDMA device

Check to make sure that ctx->cm_id->device is set before we use it.
Otherwise userspace can trigger a NULL dereference by doing
RDMA_USER_CM_CMD_SET_OPTION on an ID that is not bound to a device.

Cc: <stable@vger.kernel.org>
Reported-by:  <syzbot+a67bc93e14682d92fc2f@syzkaller.appspotmail.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>