udp: Add lockless transmit path

The UDP transmit path has been running under the socket lock
for a long time because of the corking feature.  This means that
transmitting to the same socket in multiple threads does not
scale at all.

However, as most users don't actually use corking, the locking
can be removed in the common case.

This patch creates a lockless fast path where corking is not used.

Please note that this does create a slight inaccuracy in the
enforcement of socket send buffer limits.  In particular, we
may exceed the socket limit by up to (number of CPUs) * (packet
size) because of the way the limit is computed.

As the primary purpose of socket buffers is to indicate congestion,
this should not be a great problem for now.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>