l2tp: make local function static

Also moved the refcound inlines from l2tp_core.h to l2tp_core.c
since only used in that one file.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>