The primary mbuf cluster management code needs to be made MPSAFE since

The primary mbuf cluster management code needs to be made MPSAFE since
m_free() is now MPSAFE.  This involves using atomic_*() increment and
decrement for the mbuf cluster ref count and using a serializer if there
is a potential for a double-free (if the ref count is 2 or greater).  The
common code path will tend not to need the serializer unless the interface
is being tapped.

Reported-by: Peter Avalos <pavalos@theshell.com>