net/smc: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

There are currently a couple of objects in `struct smc_clc_msg_proposal_area`
that contain a couple of flexible structures:

struct smc_clc_msg_proposal_area {
...
struct smc_clc_v2_extension pclc_v2_ext;
...
struct smc_clc_smcd_v2_extension pclc_smcd_v2_ext;
...
};

So, in order to avoid ending up with a couple of flexible-array members
in the middle of a struct, we use the `struct_group_tagged()` helper to
separate the flexible array from the rest of the members in the flexible
structure:

struct smc_clc_smcd_v2_extension {
struct_group_tagged(smc_clc_smcd_v2_extension_fixed, fixed,
u8 system_eid[SMC_MAX_EID_LEN];
u8 reserved[16];
);
struct smc_clc_smcd_gid_chid gidchid[];
};

With the change described above, we now declare objects of the type of
the tagged struct without embedding flexible arrays in the middle of
another struct:

struct smc_clc_msg_proposal_area {
...
struct smc_clc_v2_extension_fixed pclc_v2_ext;
...
struct smc_clc_smcd_v2_extension_fixed pclc_smcd_v2_ext;
...
};

We also use `container_of()` when we need to retrieve a pointer to the
flexible structures.

So, with these changes, fix the following warnings:

In file included from net/smc/af_smc.c:42:
net/smc/smc_clc.h:186:49: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
186 | struct smc_clc_v2_extension pclc_v2_ext;
| ^~~~~~~~~~~
net/smc/smc_clc.h:188:49: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
188 | struct smc_clc_smcd_v2_extension pclc_smcd_v2_ext;
| ^~~~~~~~~~~~~~~~

Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Wen Gu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/smc/smc_clc.c

···
 	pclc_smcd = &pclc->pclc_smcd;
 	pclc_prfx = &pclc->pclc_prfx;
 	ipv6_prfx = pclc->pclc_prfx_ipv6;
-	v2_ext = &pclc->pclc_v2_ext;
-	smcd_v2_ext = &pclc->pclc_smcd_v2_ext;
+	v2_ext = container_of(&pclc->pclc_v2_ext,
+			      struct smc_clc_v2_extension, fixed);
+	smcd_v2_ext = container_of(&pclc->pclc_smcd_v2_ext,
+				   struct smc_clc_smcd_v2_extension, fixed);
 	gidchids = pclc->pclc_gidchids;
 	trl = &pclc->pclc_trl;
 

net/smc/smc_clc.h

···
 			 */
 
 struct smc_clc_v2_extension {
-	struct smc_clnt_opts_area_hdr hdr;
-	u8 roce[16];		/* RoCEv2 GID */
-	u8 max_conns;
-	u8 max_links;
-	__be16 feature_mask;
-	u8 reserved[12];
+	/* New members must be added within the struct_group() macro below. */
+	struct_group_tagged(smc_clc_v2_extension_fixed, fixed,
+		struct smc_clnt_opts_area_hdr hdr;
+		u8 roce[16];		/* RoCEv2 GID */
+		u8 max_conns;
+		u8 max_links;
+		__be16 feature_mask;
+		u8 reserved[12];
+	);
 	u8 user_eids[][SMC_MAX_EID_LEN];
 };
 
···
 };
 
 struct smc_clc_smcd_v2_extension {
-	u8 system_eid[SMC_MAX_EID_LEN];
-	u8 reserved[16];
+	/* New members must be added within the struct_group() macro below. */
+	struct_group_tagged(smc_clc_smcd_v2_extension_fixed, fixed,
+		u8 system_eid[SMC_MAX_EID_LEN];
+		u8 reserved[16];
+	);
 	struct smc_clc_smcd_gid_chid gidchid[];
 };
 
···
 	struct smc_clc_msg_smcd			pclc_smcd;
 	struct smc_clc_msg_proposal_prefix	pclc_prfx;
 	struct smc_clc_ipv6_prefix	pclc_prfx_ipv6[SMC_CLC_MAX_V6_PREFIX];
-	struct smc_clc_v2_extension		pclc_v2_ext;
+	struct smc_clc_v2_extension_fixed	pclc_v2_ext;
 	u8			user_eids[SMC_CLC_MAX_UEID][SMC_MAX_EID_LEN];
-	struct smc_clc_smcd_v2_extension	pclc_smcd_v2_ext;
+	struct smc_clc_smcd_v2_extension_fixed	pclc_smcd_v2_ext;
 	struct smc_clc_smcd_gid_chid
 				pclc_gidchids[SMCD_CLC_MAX_V2_GID_ENTRIES];
 	struct smc_clc_msg_trail		pclc_trl;