sys/net: move DOT1Q_VID_* constants to ethernet.h

These are generally useful to anything dealing with 802.1q and aren't
specific to if_vlan, so move them to <net/ethernet.h> and remove the
_KERNEL gate.

While here, document what they actually mean.

Reviewed by:	kp, des
Approved by:	des (mentor)
Differential Revision:	https://reviews.freebsd.org/D50570

sys/net/ethernet.h

@@ -80,6 +80,23 @@ struct ether_addr {
 	(((addr)[0] | (addr)[1] | (addr)[2] | \
 	  (addr)[3] | (addr)[4] | (addr)[5]) == 0x00)
 
+/*
+ * 802.1q VID constants from IEEE 802.1Q-2014, table 9-2.
+ */
+
+/* Null VID: The tag contains only PCP (priority) and DEI information. */
+#define	DOT1Q_VID_NULL		0x0
+/* The default PVID for a bridge port.  NB: bridge(4) does not honor this. */
+#define	DOT1Q_VID_DEF_PVID	0x1
+/* The default SR_PVID for SRP Stream related traffic. */
+#define	DOT1Q_VID_DEF_SR_PVID	0x2
+/* A VID reserved for implementation use, not permitted on the wire. */
+#define	DOT1Q_VID_RSVD_IMPL	0xfff
+/* The lowest valid VID. */
+#define	DOT1Q_VID_MIN		0x1
+/* The highest valid VID. */
+#define	DOT1Q_VID_MAX		0xffe
+
 /*
  * This is the type of the VLAN ID inside the tag, not the tag itself.
  */

sys/net/if_vlan_var.h

@@ -126,13 +126,6 @@ struct	vlanreq {
 
 #define	VLAN_PCP_MAX		7
 
-#define	DOT1Q_VID_NULL		0x0
-#define	DOT1Q_VID_DEF_PVID	0x1
-#define	DOT1Q_VID_DEF_SR_PVID	0x2
-#define	DOT1Q_VID_RSVD_IMPL	0xfff
-#define	DOT1Q_VID_MIN		1	/* minimum valid vlan id */
-#define	DOT1Q_VID_MAX		4094	/* maximum valid vlan id */
-
 /*
  * 802.1q full tag. Proto and vid are stored in host byte order.
  */