mbuf(9): Restrict m_copyback() to forbid mbuf expansion

mbuf(9): Restrict m_copyback() to forbid mbuf expansion

Most uses of m_copyback() are in-place updates to a portion (e.g., the
header) of a packet, so no mbuf allocation is required.  It's more clear
to make m_copyback() forbid mbuf expansion instead of let it implicitly
do allocation with how=M_NOWAIT.  Due to the lack of a return value, the
caller is hard to identify an m_copyback() failure, so diagnostics info
will be printed to the console in that case.

Change the original m_copyback2() to be an inline function, and
implement both m_copyback() and m_copyback2() as wrappers of it.

Bump __DragonFly_version for this behavior change.

Discussed-with: dillon