RDMA/cxgb4: Replace 0-length arrays with flexible arrays

Zero-length arrays are deprecated[1]. Replace all remaining
0-length arrays with flexible arrays. Detected with GCC 13, using
-fstrict-flex-arrays=3:

In function 'build_rdma_write',
    inlined from 'c4iw_post_send' at ../drivers/infiniband/hw/cxgb4/qp.c:1173:10:
../drivers/infiniband/hw/cxgb4/qp.c:597:38: warning: array subscript 0 is outside array bounds of 'struct fw_ri_immd[0]' [-Warray-bounds=]
  597 |                 wqe->write.u.immd_src[0].r2 = 0;
      |                 ~~~~~~~~~~~~~~~~~~~~~^~~
../drivers/infiniband/hw/cxgb4/t4fw_ri_api.h: In function 'c4iw_post_send':
../drivers/infiniband/hw/cxgb4/t4fw_ri_api.h:567:35: note: while referencing 'immd_src'
  567 |                 struct fw_ri_immd immd_src[0];
      |                                   ^~~~~~~~

Additionally drop the unused C99_NOT_SUPPORTED ifndef lines.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: Potnuri Bharat Teja <bharat@chelsio.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230105223225.never.252-kees@kernel.orgReviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/hw/cxgb4/t4fw_ri_api.h

@@ -122,9 +122,7 @@ struct fw_ri_dsgl {
 	__be16	nsge;
 	__be32	len0;
 	__be64	addr0;
-#ifndef C99_NOT_SUPPORTED
 	struct fw_ri_dsge_pair sge[];
-#endif
 };
 
 struct fw_ri_sge {
@@ -138,9 +136,7 @@ struct fw_ri_isgl {
 	__u8	r1;
 	__be16	nsge;
 	__be32	r2;
-#ifndef C99_NOT_SUPPORTED
 	struct fw_ri_sge sge[];
-#endif
 };
 
 struct fw_ri_immd {
@@ -148,9 +144,7 @@ struct fw_ri_immd {
 	__u8	r1;
 	__be16	r2;
 	__be32	immdlen;
-#ifndef C99_NOT_SUPPORTED
 	__u8	data[];
-#endif
 };
 
 struct fw_ri_tpte {
@@ -320,9 +314,7 @@ struct fw_ri_res_wr {
 	__be32 op_nres;
 	__be32 len16_pkd;
 	__u64  cookie;
-#ifndef C99_NOT_SUPPORTED
 	struct fw_ri_res res[];
-#endif
 };
 
 #define FW_RI_RES_WR_NRES_S	0
@@ -562,12 +554,10 @@ struct fw_ri_rdma_write_wr {
 	__be32 plen;
 	__be32 stag_sink;
 	__be64 to_sink;
-#ifndef C99_NOT_SUPPORTED
 	union {
-		struct fw_ri_immd immd_src[0];
-		struct fw_ri_isgl isgl_src[0];
+		DECLARE_FLEX_ARRAY(struct fw_ri_immd, immd_src);
+		DECLARE_FLEX_ARRAY(struct fw_ri_isgl, isgl_src);
 	} u;
-#endif
 };
 
 struct fw_ri_send_wr {
@@ -581,12 +571,10 @@ struct fw_ri_send_wr {
 	__be32 plen;
 	__be32 r3;
 	__be64 r4;
-#ifndef C99_NOT_SUPPORTED
 	union {
-		struct fw_ri_immd immd_src[0];
-		struct fw_ri_isgl isgl_src[0];
+		DECLARE_FLEX_ARRAY(struct fw_ri_immd, immd_src);
+		DECLARE_FLEX_ARRAY(struct fw_ri_isgl, isgl_src);
 	} u;
-#endif
 };
 
 #define FW_RI_SEND_WR_SENDOP_S		0
@@ -618,12 +606,10 @@ struct fw_ri_rdma_write_cmpl_wr {
 		struct fw_ri_isgl isgl_src;
 	} u_cmpl;
 	__be64 r3;
-#ifndef C99_NOT_SUPPORTED
 	union fw_ri_write {
-		struct fw_ri_immd immd_src[0];
-		struct fw_ri_isgl isgl_src[0];
+		DECLARE_FLEX_ARRAY(struct fw_ri_immd, immd_src);
+		DECLARE_FLEX_ARRAY(struct fw_ri_isgl, isgl_src);
 	} u;
-#endif
 };
 
 struct fw_ri_rdma_read_wr {