Handle read-only mbufs in iflib ether pad function

If ethernet padding is enabled, and a read-only mbuf is passed,
it would modify the mbuf using m_append(). Instead, call m_dup() and
append to the new packet.

Reported by: Pyun YongHyeon
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D13414