xfrm: Remove unneeded device check from validate_xmit_xfrm

validate_xmit_xfrm checks whether a packet already passed through it on
the master device (xso.dev) and skips processing the skb again on the
slave device (xso.real_dev).

This check was added in commit [1] to avoid tx packets on a bond device
pass through xfrm twice and get two sets of headers, but the check was
soon obsoleted by commit [2], which was added around the same time to
fix a similar but unrelated problem. Commit [3] set XFRM_XMIT only when
packets are hw offloaded.

xso.dev is usually equal to xso.real_dev, unless bonding is used, in
which case the bonding driver uses xso.real_dev to manage offloaded xfrm
states.

Since commit [3], the check added in commit [1] is unused on all cases,
since packets going through validate_xmit_xfrm twice bail out on the
check added in commit [2]. Here's a breakdown of relevant scenarios:

1. ESP offload off: validate_xmit_xfrm returns early on !xo.
2. ESP offload on, no bond: skb->dev == xso.real_dev == xso.dev.
3. ESP offload on, bond, xs on bond dev: 1st pass adds XFRM_XMIT, 2nd
   pass returns early on XFRM_XMIT.
3. ESP offload on, bond, xs on slave dev: 1st pass returns early on
   !xo, 2nd pass adds XFRM_XMIT.
4. ESP offload on, bond, xs on both bond AND slave dev: only 1 offload
   possible in secpath. Either 1st pass adds XFRM_XMIT and 2nd pass returns
   early on XFRM_XMIT, or 1st pass is sw and returns early on !xo.
6. ESP offload on, crypto fallback triggered in esp_xmit/esp6_xmit: 1st
   pass does sw crypto & secpath_reset, 2nd pass returns on !xo.

This commit removes the unnecessary check, so xso.real_dev becomes what
it is in practice: a private field managed by bonding driver.
The check immediately below that can be simplified as well.

[1] commit 272c2330 ("xfrm: bail early on slave pass over skb")
[2] commit 94579ac3 ("xfrm: Fix double ESP trailer insertion in
IPsec crypto offload.")
[3] commit c7dbf4c0 ("xfrm: Provide private skb extensions for
segmented and hw offloaded ESP packets")

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>