In ip_output(), do not acquire the IN_MULTI_LOCK(),

In ip_output(), do not acquire the IN_MULTI_LOCK(),
and do not attempt to perform a group lookup.
This is a socket layer lock, and the bottom half of IP
really has no business taking it.

Use the value of the in_mcast_loop sysctl to determine
if we should loop back by default, in the absence of
any multicast socket options. Because the check on
group membership is now deferred to the input path,
an m_copym() is now required.

This should increase multicast send performance where the
source has not requested loopback, although this has not been
benchmarked or measured.

It is also a necessary change for IN_MULTI_LOCK to become
non-recursive, which is required in order to implement IGMPv3
in a thread-safe way.