RDMA/mlx5: Use memset_after() to zero struct mlx5_ib_mr

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Use memset_after() to zero the end of struct mlx5_ib_mr that should be
initialized.

Link: https://lore.kernel.org/r/20211213223331.135412-10-keescook@chromium.orgAcked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/mlx5_ib.h

@@ -664,8 +664,8 @@ struct mlx5_ib_mr {
 
 	/* User MR data */
 	struct mlx5_cache_ent *cache_ent;
+	/* Everything after cache_ent is zero'd when MR allocated */
 
-	/* This is zero'd when the MR is allocated */
 	union {
 		/* Used only while the MR is in the cache */
 		struct {
@@ -718,7 +718,7 @@ struct mlx5_ib_mr {
 /* Zero the fields in the mr that are variant depending on usage */
 static inline void mlx5_clear_mr(struct mlx5_ib_mr *mr)
 {
-	memset(mr->out, 0, sizeof(*mr) - offsetof(struct mlx5_ib_mr, out));
+	memset_after(mr, 0, cache_ent);
 }
 
 static inline bool is_odp_mr(struct mlx5_ib_mr *mr)