net/udp: Update udp_encap_needed static key to modern api

No changes in refcount semantics -- key init is false; replace

static_key_enable         with   static_branch_enable
static_key_slow_inc|dec   with   static_branch_inc|dec
static_key_false          with   static_branch_unlikely

Added a '_key' suffix to udp and udpv6 encap_needed, for better
self documentation.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>