Partially revert "net/smc: Add netlink net namespace support"

The change of sizeof(struct smc_diag_linkinfo) by commit 79d39fc
("net/smc: Add netlink net namespace support") introduced an ABI
regression: since struct smc_diag_lgrinfo contains an object of
type "struct smc_diag_linkinfo", offset of all subsequent members
of struct smc_diag_lgrinfo was changed by that change.

As result, applications compiled with the old version
of struct smc_diag_linkinfo will receive garbage in
struct smc_diag_lgrinfo.role if the kernel implements
this new version of struct smc_diag_linkinfo.

Fix this regression by reverting the part of commit 79d39fc that
changes struct smc_diag_linkinfo.  After all, there is SMC_GEN_NETLINK
interface which is good enough, so there is probably no need to touch
the smc_diag ABI in the first place.

Fixes: 79d39fc ("net/smc: Add netlink net namespace support")
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Reviewed-by: Karsten Graul <kgraul@linux.ibm.com>
Link: https://lore.kernel.org/r/20220202030904.GA9742@altlinux.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/smc_diag.h

@@ -84,12 +84,11 @@ struct smc_diag_conninfo {
 /* SMC_DIAG_LINKINFO */
 
 struct smc_diag_linkinfo {
-	__u8		link_id;		    /* link identifier */
-	__u8		ibname[IB_DEVICE_NAME_MAX]; /* name of the RDMA device */
-	__u8		ibport;			    /* RDMA device port number */
-	__u8		gid[40];		    /* local GID */
-	__u8		peer_gid[40];		    /* peer GID */
-	__aligned_u64	net_cookie;                 /* RDMA device net namespace */
+	__u8 link_id;			/* link identifier */
+	__u8 ibname[IB_DEVICE_NAME_MAX]; /* name of the RDMA device */
+	__u8 ibport;			/* RDMA device port number */
+	__u8 gid[40];			/* local GID */
+	__u8 peer_gid[40];		/* peer GID */
 };
 
 struct smc_diag_lgrinfo {

net/smc/smc_diag.c

@@ -146,13 +146,11 @@ static int __smc_diag_dump(struct sock *sk, struct sk_buff *skb,
 	    (req->diag_ext & (1 << (SMC_DIAG_LGRINFO - 1))) &&
 	    !list_empty(&smc->conn.lgr->list)) {
 		struct smc_link *link = smc->conn.lnk;
-		struct net *net = read_pnet(&link->smcibdev->ibdev->coredev.rdma_net);
 
 		struct smc_diag_lgrinfo linfo = {
 			.role = smc->conn.lgr->role,
 			.lnk[0].ibport = link->ibport,
 			.lnk[0].link_id = link->link_id,
-			.lnk[0].net_cookie = net->net_cookie,
 		};
 
 		memcpy(linfo.lnk[0].ibname,