fwip: Fix M_PKTHDR loss in fwip_async_output broadcast path

M_PREPEND in the broadcast branch may call m_prepend(9) which allocates
a new head mbuf and calls m_move_pkthdr(), stripping M_PKTHDR from the
old mbuf.

xfer->mbuf was set before M_PREPEND, so it pointed at the
deheadered old mbuf. bus_dmamap_load_mbuf(9) asserts M_PKTHDR and
panics.

Reviewed by: zlei, adrian
Differential Revision: https://reviews.freebsd.org/D57495