ethtool: Replace 0-length array with flexible array

Zero-length arrays are deprecated[1]. Replace struct ethtool_rxnfc's
"rule_locs" 0-length array with a flexible array. Detected with GCC 13,
using -fstrict-flex-arrays=3:

net/ethtool/common.c: In function 'ethtool_get_max_rxnfc_channel':
net/ethtool/common.c:558:55: warning: array subscript i is outside array bounds of '__u32[0]' {aka 'unsigned int[]'} [-Warray-bounds=]
  558 |                         .fs.location = info->rule_locs[i],
      |                                        ~~~~~~~~~~~~~~~^~~
In file included from include/linux/ethtool.h:19,
                 from include/uapi/linux/ethtool_netlink.h:12,
                 from include/linux/ethtool_netlink.h:6,
                 from net/ethtool/common.c:3:
include/uapi/linux/ethtool.h:1186:41: note: while referencing
'rule_locs'
 1186 |         __u32                           rule_locs[0];
      |                                         ^~~~~~~~~

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

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: kernel test robot <lkp@intel.com>
Cc: Oleksij Rempel <linux@rempel-privat.de>
Cc: Sean Anderson <sean.anderson@seco.com>
Cc: Alexandru Tachici <alexandru.tachici@analog.com>
Cc: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230106042844.give.885-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/ethtool.h

@@ -1183,7 +1183,7 @@ struct ethtool_rxnfc {
 		__u32			rule_cnt;
 		__u32			rss_context;
 	};
-	__u32				rule_locs[0];
+	__u32				rule_locs[];
 };