esp: Consolidate esp4 and esp6

This patch merges common code of esp4.c and esp6.c into
xfrm_esp.c. This almost halves the size of the ESP
implementation for the price of three indirect calls
on UDP/TCP encapsulation. No functional changes.

Changes from the RFC version:

- Fix a typo in the commit message.

- Remove some old comments that don't make sense anymore.

- Let the ->input_encap functions return the needed offsets.

- Remove the IP_MAX_MTU check from UDP/TCP encap.
  The IPv4/IPv6 local_out function will do that ceck later.

- The comment on IPv4 ESP offload with UDP encapsulation
  is true for IPv4 and IPv6, so remove the IPv4 from the
  comment.

Changes since v1:

- Remove some now unused code.

- Whitespace fixes.

- Cleanup the header length calculation in the input path for
  UDP/TCP encapsulation.

- Move the skb_pull_rcsum() call to the generic esp_input_done2()
  function.

- Move the skb_postpull_rcsum() call to the offloading codepath,
  it is needed only there.

Changes since v2:

- Use CONFIG_XFRM_ESPINTCP instead of CONFIG_INET_ESPINTCP
  to compile the common code for ESP4 and ESP6.

- Re-add the length check when building UDP/TCP encap
  packets.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Tested-by: Tobias Brunner <tobias@strongswan.org>