l2tp: remove inline from functions in c sources

Update l2tp to remove the inline keyword from several functions in C
sources, since this is now discouraged.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>