Don't register IOCTLs with capsicum when there is no valid file descriptor.

This fixes tcpdump when using mlx5_X devices.

Differential Revision:	https://reviews.freebsd.org/D18499
Reviewed by:		kib@, slavash@, oshogbo@
MFC after:		1 week
Sponsored by:		Mellanox Technologies

contrib/tcpdump/tcpdump.c

@@ -1915,7 +1915,7 @@ main(int argc, char **argv)
 	if (pcap_setfilter(pd, &fcode) < 0)
 		error("%s", pcap_geterr(pd));
 #ifdef HAVE_CAPSICUM
-	if (RFileName == NULL && VFileName == NULL) {
+	if (RFileName == NULL && VFileName == NULL && pcap_fileno(pd) != -1) {
 		static const unsigned long cmds[] = { BIOCGSTATS, BIOCROTZBUF };
 
 		/*