In mb_dupcl() don't copy full m_ext, to avoid cache miss. Respectively,
in mb_free_ext() always use fields from the original refcount holding
mbuf (see. r296242) mbuf. Cuts another cache miss from mb_free_ext().

However, treat EXT_EXTREF mbufs differently, since they are different -
they don't have a refcount holding mbuf.

Provide longer comments in m_ext declaration to explain this change
and change from r296242.

In collaboration with: gallatin
Differential Revision: https://reviews.freebsd.org/D12615