RDMA/mlx5: Use a union inside mlx5_ib_mr

The struct mlx5_ib_mr can be used for three different things, but only one
at a time:

 - In the user MR cache
 - As a kernel MR
 - As a user MR

Overlay the three things into a single union with the following rules:

 - If the mr is found on the cache_ent->head list then it is a cache MR
   and umem == NULL. The entire union is zero after the MR is removed from
   the cache.

 - If umem != NULL or type == IB_MR_TYPE_USER then it is a user MR.

 - If umem == NULL then it is a kernel MR

This reduces the size of struct mlx5_ib_mr to 552 bytes from 702.

The only place the three flows overlap in the code is during dereg, so add
a few extra checks along there.

Link: https://lore.kernel.org/r/20210304120745.1090751-3-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/mlx5_ib.h

@@ -657,7 +657,7 @@ struct mlx5_ib_mr {
 	struct ib_umem *umem;
 
 	/* This is zero'd when the MR is allocated */
-	struct {
+	union {
 		/* Used only while the MR is in the cache */
 		struct {
 			u32 out[MLX5_ST_SZ_DW(create_mkey_out)];

drivers/infiniband/hw/mlx5/mr.c

@@ -1929,7 +1929,7 @@ mlx5_alloc_priv_descs(struct ib_device *device,
 static void
 mlx5_free_priv_descs(struct mlx5_ib_mr *mr)
 {
-	if (mr->descs) {
+	if (!mr->umem && mr->descs) {
 		struct ib_device *device = mr->ibmr.device;
 		int size = mr->max_descs * mr->desc_size;
 		struct mlx5_ib_dev *dev = to_mdev(device);
@@ -1943,7 +1943,7 @@ mlx5_free_priv_descs(struct mlx5_ib_mr *mr)
 
 static void clean_mr(struct mlx5_ib_dev *dev, struct mlx5_ib_mr *mr)
 {
-	if (mr->sig) {
+	if (mr->ibmr.type == IB_MR_TYPE_INTEGRITY) {
 		if (mlx5_core_destroy_psv(dev->mdev,
 					  mr->sig->psv_memory.psv_idx))
 			mlx5_ib_warn(dev, "failed to destroy mem psv %d\n",