udp: implement complete book-keeping for encap_needed

The *encap_needed static keys are enabled by UDP tunnels
and several UDP encapsulations type, but they are never
turned off. This can cause unneeded overall performance
degradation for systems where such features are used
transiently.

This patch introduces complete book-keeping for such keys,
decreasing the usage at socket destruction time, if needed,
and avoiding that the same socket could increase the key
usage multiple times.

rfc v3 -> v1:
 - add socket lock around udp_tunnel_encap_enable()

rfc v2 -> rfc v3:
 - use udp_tunnel_encap_enable() in setsockopt()

Change-Id: Ie2faefd68ef38dd40494046b9db481044b7c5b3a
CRs-Fixed: 2351793
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git


Git-commit: 60fb9567bf30937e6bedfa939d7c8fd4ee6a1b1c
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>