if_tuntap: Enable MEXTPG support

Fix tunread() to use m_mbuftouio() instead of manually copying (which
doesn't work for unmapped mbufs).

Reviewed by: jhb, gallatin
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47295

(cherry picked from commit 01c738cd5c3938374cce8293c82753d977966154)