platform: detect kernel support for FRA_L3MDEV

(cherry picked from commit eba4fd56f5)

src/platform/nm-linux-platform.c

@@ -3402,6 +3402,16 @@ _new_from_nl_routing_rule (struct nlmsghdr *nlh, gboolean id_only)
 		props->tun_id = nla_get_be64 (tb[FRA_TUN_ID]);
 
 	if (tb[FRA_L3MDEV]) {
+
+		if (!_nm_platform_kernel_support_detected (NM_PLATFORM_KERNEL_SUPPORT_TYPE_FRA_L3MDEV)) {
+			/* support for FRA_L3MDEV was added in 96c63fa7393d0a346acfe5a91e0c7d4c7782641b,
+			 * kernel 4.8, 3 October 2017.
+			 *
+			 * We can only detect support if the attribute is present. A missing attribute
+			 * is not conclusive. */
+			_nm_platform_kernel_support_init (NM_PLATFORM_KERNEL_SUPPORT_TYPE_FRA_L3MDEV, 1);
+		}
+
 		/* actually, kernel only allows this attribute to be missing or
 		 * "1". Still, encode it as full uint8.
 		 *

src/platform/nm-platform.c

@@ -296,6 +296,11 @@ static const struct {
 		.name                 = "RTA_PREF",
 		.desc                 = "ability to set router preference for IPv6 routes",
 	},
+	[NM_PLATFORM_KERNEL_SUPPORT_TYPE_FRA_L3MDEV] = {
+		.compile_time_default = (FRA_MAX >= 19 /* FRA_L3MDEV */),
+		.name                 = "FRA_L3MDEV",
+		.desc                 = "FRA_L3MDEV attribute for policy routing rules",
+	},
 	[NM_PLATFORM_KERNEL_SUPPORT_TYPE_FRA_UID_RANGE] = {
 		.compile_time_default = (FRA_MAX >= 20 /* FRA_UID_RANGE */),
 		.name                 = "FRA_UID_RANGE",

src/platform/nm-platform.h

@@ -848,6 +848,7 @@ typedef enum {
 	NM_PLATFORM_KERNEL_SUPPORT_TYPE_EXTENDED_IFA_FLAGS,
 	NM_PLATFORM_KERNEL_SUPPORT_TYPE_USER_IPV6LL,
 	NM_PLATFORM_KERNEL_SUPPORT_TYPE_RTA_PREF,
+	NM_PLATFORM_KERNEL_SUPPORT_TYPE_FRA_L3MDEV,
 	NM_PLATFORM_KERNEL_SUPPORT_TYPE_FRA_UID_RANGE,
 	NM_PLATFORM_KERNEL_SUPPORT_TYPE_FRA_PROTOCOL,