MFC r368326: kern: soclose: don't sleep on SO_LINGER w/ timeout=0

This is a valid scenario that's handled in the various protocol layers where
it makes sense (e.g., tcp_disconnect and sctp_disconnect). Given that it
indicates we should immediately drop the connection, it makes little sense
to sleep on it.

This could lead to panics with INVARIANTS. On non-INVARIANTS kernels, this
could result in the thread hanging until a signal interrupts it if the
protocol does not mark the socket as disconnected for whatever reason.