bpf: Fix the write filter

[markj]

Summary

A BPF descriptor only has an associated interface descriptor once it is
attached to an interface, e.g., with BIOCSETIF. Avoid dereferencing a
NULL pointer in filt_bpfwrite() if the BPF descriptor is not attached.

Fixes: ded77e0237a8 ("Allow the BPF to be select for write.")
Reported by: [e-mail]

[ae]

LGTM.

lib/libc/sys/kqueue.2

  * defined only between the device driver and VLAN layer.
  */
 #define MTAG_8021Q              1326104895
 #define MTAG_8021Q_PCP_IN       0               /* Input priority. */
 #define MTAG_8021Q_PCP_OUT      1               /* Output priority. */
 
 #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
+
 /*
  * 802.1q full tag. Proto and vid are stored in host byte order.
  */
 struct ether_8021q_tag {
         uint16_t proto;
         uint16_t vid;
         uint8_t  pcp;
 };

sys/net/bpf.c

  * defined only between the device driver and VLAN layer.
  */
 #define MTAG_8021Q              1326104895
 #define MTAG_8021Q_PCP_IN       0               /* Input priority. */
 #define MTAG_8021Q_PCP_OUT      1               /* Output priority. */
 
 #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
+
 /*
  * 802.1q full tag. Proto and vid are stored in host byte order.
  */
 struct ether_8021q_tag {
         uint16_t proto;
         uint16_t vid;
         uint8_t  pcp;
 };