mbuf: Make m_unshare() fail on KTLS mbufs

Commit f2202ab5abda did not account for KTLS mbufs.

m_unshare() tries to linearize the original mbuf chain and creates a
writable copy of it, converting unmapped mbufs. Both of them are
unsafe for KTLS mbufs.

It is better to return NULL if the mbuf chain contains a KTLS mbuf.

Reported by: jhb
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D58466