mbuf: make m_unshare() allow unmapped mbufs

m_unshare() had crashed if unmapped mbufs exist in the mbuf chain.
This was because memcpy() with mtod() was used without making sure that
the mbuf was mapped. Use m_copydata() that cares unmapped mbufs instead.

Reviewed by: gallatin
Differential Revision: https://reviews.freebsd.org/D58189