tree-wide: Always include <net/if.h> before related linux headers

Otherwise, we get redefinition errors if <net/if.h> is included later
on by another header.

src/libsystemd-network/sd-dhcp6-client.c

@@ -5,8 +5,9 @@
 
 #include <errno.h>
 #include <sys/ioctl.h>
-#include <linux/if_arp.h>
-#include <linux/if_infiniband.h>
+#include <net/if.h>
+#include <linux/if_arp.h> /* Must be included after <net/if.h> */
+#include <linux/if_infiniband.h> /* Must be included after <net/if.h> */
 
 #include "sd-dhcp6-client.h"
 

src/network/networkd-dhcp-server.c

@@ -1,8 +1,9 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <netinet/in.h>
-#include <linux/if_arp.h>
-#include <linux/if.h>
+#include <net/if.h>
+#include <linux/if_arp.h> /* Must be included after <net/if.h> */
+#include <linux/if.h> /* Must be included after <net/if.h> */
 
 #include "sd-dhcp-server.h"
 

src/network/networkd-manager.c

@@ -3,7 +3,8 @@
 #include <netinet/in.h>
 #include <sys/socket.h>
 #include <unistd.h>
-#include <linux/if.h>
+#include <net/if.h>
+#include <linux/if.h> /* Must be included after <net/if.h> */
 #include <linux/fib_rules.h>
 #include <linux/nexthop.h>
 #include <linux/nl80211.h>

src/network/networkd-state-file.c

@@ -1,7 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <netinet/in.h>
-#include <linux/if.h>
+#include <net/if.h>
+#include <linux/if.h> /* Must be included after <net/if.h> */
 
 #include "alloc-util.h"
 #include "dns-domain.h"

src/resolve/resolved-link.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
-#include <linux/if.h>
+#include <net/if.h>
+#include <linux/if.h> /* Must be included after <net/if.h> */
 #include <unistd.h>
 
 #include "sd-network.h"

src/udev/net/link-config.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
-#include <linux/netdevice.h>
+#include <net/if.h>
+#include <linux/netdevice.h> /* Must be included after <net/if.h> */
 #include <netinet/ether.h>
 #include <unistd.h>