[XFRM] netlink: Inline attach_encap_tmpl(), attach_sec_ctx(), and attach_one_addr()

These functions are only used once and are a lot easier to understand if
inlined directly into the function.

Fixes by Masahide NAKAMURA.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>