Clear external storage flags in m_extaddref().

[jhb]

Summary

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().

Test Plan

• I'm using this in a feature branch where I use EXT_FLAG_VENDOR1 in conjuntion with m_extaddref().

[jhb]

Closed by commit rS280222 (authored by @jhb).