ipfw: Update the pfil mbuf pointer in ipfw_check_frame()
    
    ipfw_chk() might call m_pullup() and thus can change the mbuf chain
    head.  In this case, the new chain head has to be returned to the pfil
    hook caller, otherwise the pfil hook caller is left with a dangling
    pointer.
    
    Note that this affects only the link-layer hooks installed when the
    net.link.ether.ipfw sysctl is set to 1.
    
    PR:             256439, 254015, 255069, 255104
    Fixes:          f355cb3e6
    Reviewed by:    ae
    MFC after:      3 days
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D30764
--- sys/netpfil/ipfw/ip_fw_pfil.c.orig
+++ sys/netpfil/ipfw/ip_fw_pfil.c
@@ -371,6 +371,8 @@
 	}
 
 	ipfw = ipfw_chk(&args);
+	if (!mem)
+		*p.m = args.m;
 
 	ret = PFIL_PASS;
 	switch (ipfw) {