Release mbuf(9) chain with a simple m_freem(9) loop in sorflush().

Release mbuf(9) chain with a simple m_freem(9) loop in sorflush().
Passing local copy of socket to sbrelease() is too complicated to just
free receive buffer. We don't allocate large object on the stack. Also
we don't pass unlocked socket to soassertlocked() within sbdrop(). This
was not triggered because we lock the whole layer with one lock.

Also sorflush() is now private to kern/uipc_socket.c, so it's definition
was made to be in accordance.

ok claudio@ mpi@