[IPSEC]: Rename tunnel-mode functions to avoid collisions with tunnels

It appears that I've managed to create two different functions both
called xfrm6_tunnel_output.  This is because we have the plain tunnel
encapsulation named xfrmX_tunnel as well as the tunnel-mode encapsulation
which lives in the files xfrmX_mode_tunnel.c.

This patch renames functions from the latter to use the xfrmX_mode_tunnel
prefix to avoid name-space conflicts.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>