When the kernel is compiled with options IPXIP, run the network stack

with Giant, as there is current unsafety in the IPX tunneled over IP
code.  There have been no reports of trouble, but there probably would
be if anyone were running this code at high speed on SMP systems.

MFC after:	3 days

sys/netipx/ipx_ip.c

@@ -75,6 +75,8 @@ __FBSDID("$FreeBSD$");
 #include <netipx/ipx_ip.h>
 #include <netipx/ipx_var.h>
 
+NET_NEEDS_GIANT("ipx_ip");
+
 static struct	ifnet ipxipif;
 static int	ipxipif_units;