xfrm: remove get_mtu indirection from xfrm_type

esp4_get_mtu and esp6_get_mtu are exactly the same, the only difference
is a single sizeof() (ipv4 vs. ipv6 header).

Merge both into xfrm_state_mtu() and remove the indirection.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>