bridge: Fix fragment handling and memory leak

Fragmented UDP and ICMP packets were corrupted if a firewall with reassembling
feature (like pf'scrub) is enabled on the bridge. This patch fixes corrupted
packet problem and the panic (triggered easly with low RAM) as explain in PR

bridge_pfil and bridge_fragment relationship:

bridge_pfil() receive (IN direction) packets and sent it to the firewall The
firewall can be configured for reassembling fragmented packet (like pf'scrubing)
in one mbuf chain when bridge_pfil() need to send this reassembled packet to the
outgoing interface, it needs to re-fragment it by using bridge_fragment()
bridge_fragment() had to split this mbuf (using ip_fragment) first then
had to M_PREPEND each packet in the mbuf chain for adding Ethernet
header.

But M_PREPEND can sometime create a new mbuf on the begining of the mbuf chain,
then the "main" pointer of this mbuf chain should be updated and this case is
tottaly forgotten. The original bridge_fragment code (Revision 158140,
2006 April 29) came from OpenBSD, and the call to bridge_enqueue was
embedded. But on FreeBSD, bridge_enqueue() is done after bridge_fragment(),
then the original OpenBSD code can't work as-it of FreeBSD.

PR: 185633
Submitted by: Olivier Cochard-Labbé
Differential Revision: https://reviews.freebsd.org/D7780