cp might be null, calling cp->cp_conn would produce null dereference

[Simon Horman adds:]

Analysis:

* cp is a parameter of __rds_rdma_map and is not reassigned.

* The following call-sites pass a NULL cp argument to __rds_rdma_map()

  - rds_get_mr()
  - rds_get_mr_for_dest

* Prior to the code above, the following assumes that cp may be NULL
  (which is indicative, but could itself be unnecessary)

	trans_private = rs->rs_transport->get_mr(
		sg, nents, rs, &mr->r_key, cp ? cp->cp_conn : NULL,
		args->vec.addr, args->vec.bytes,
		need_odp ? ODP_ZEROBASED : ODP_NOT_NEEDED);

* The code modified by this patch is guarded by IS_ERR(trans_private),
  where trans_private is assigned as per the previous point in this analysis.

  The only implementation of get_mr that I could locate is rds_ib_get_mr()
  which can return an ERR_PTR if the conn (4th) argument is NULL.

* ret is set to PTR_ERR(trans_private).
  rds_ib_get_mr can return ERR_PTR(-ENODEV) if the conn (4th) argument is NULL.
  Thus ret may be -ENODEV in which case the code in question will execute.

Conclusion:
* cp may be NULL at the point where this patch adds a check;
  this patch does seem to address a possible bug

Fixes: c055fc00 ("net/rds: fix WARNING in rds_conn_connect_if_down")
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Mahmoud Adam <mngyadam@amazon.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240326153132.55580-1-mngyadam@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>