m_clrprotoflags() is designed to cleanup M_PROTO flags when mbufs cross
protocol and facility boundaries.  However, now it cleans up only the
first mbuf, while intent clearly means all mbufs in a possible chain.

Differential Revision:	https://reviews.freebsd.org/D1352
Reviewed by:		rpaulo, gnn, adrian
Sponsored by:		Nginx, Inc.