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