mbuf: make m_unshare() allow unmapped mbufs

[takahiro.kurosawa_gmail.com]

Summary

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.

[gallatin]

I'd write this as 'mcopydata(m,off, len, mtod(..));

[takahiro.kurosawa_gmail.com]

Use m_copydata() instead of m_copyfromunmapped() as suggested by the review.

[takahiro.kurosawa_gmail.com]

In D58189#1335203, @gallatin wrote:

I'd write this as 'mcopydata(m,off, len, mtod(..));

I've updated the code and the summary text according to your comment.
It gets much simpler. Thanks for the review!