RDMA/hfi1: Prevent panic when SDMA is disabled

If the hfi1 module is loaded with HFI1_CAP_SDMA off, a call to
hfi1_write_iter() will dereference a NULL pointer and panic. A typical
stack frame is:

  sdma_select_user_engine [hfi1]
  hfi1_user_sdma_process_request [hfi1]
  hfi1_write_iter [hfi1]
  do_iter_readv_writev
  do_iter_write
  vfs_writev
  do_writev
  do_syscall_64

The fix is to test for SDMA in hfi1_write_iter() and fail the I/O with
EINVAL.

Link: https://lore.kernel.org/r/20220520183706.48973.79803.stgit@awfm-01.cornelisnetworks.comSigned-off-by: Douglas Miller <doug.miller@cornelisnetworks.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/hfi1/file_ops.c

@@ -265,6 +265,8 @@ static ssize_t hfi1_write_iter(struct kiocb *kiocb, struct iov_iter *from)
 	unsigned long dim = from->nr_segs;
 	int idx;
 
+	if (!HFI1_CAP_IS_KSET(SDMA))
+		return -EINVAL;
 	idx = srcu_read_lock(&fd->pq_srcu);
 	pq = srcu_dereference(fd->pq, &fd->pq_srcu);
 	if (!cq || !pq) {