Clear an mbuf's external storage flags in m_extaddref(). They are cle…

…ared

in other places that set the external storage type (ext_type) such as
m_cljset(), m_extadd(), mb_ctor_clust(), and vn_sendfile().

Differential Revision:	https://reviews.freebsd.org/D2080
Reviewed by:	np, glebius
MFC after:	2 weeks

sys/sys/mbuf.h

@@ -578,6 +578,7 @@ m_extaddref(struct mbuf *m, caddr_t buf, u_int size, u_int *ref_cnt,
 	m->m_ext.ext_arg1 = arg1;
 	m->m_ext.ext_arg2 = arg2;
 	m->m_ext.ext_type = EXT_EXTREF;
+	m->m_ext.ext_flags = 0;
 }
 
 static __inline uma_zone_t