m_dup: Handle unmapped mbufs as an input mbuf.

Use m_copydata() instead of a direct bcopy() when copying data out of
a source mbuf into a newly-allocated mbuf.

PR: 256610
Reported by: Niels Bakker <niels=freebsd@bakker.net>
Reviewed by: markj
MFC after: 2 weeks

(cherry picked from commit be79f30d6c3e353856d4f82227b270abc26be702)