netdevsim: drop PSP ext ref on forward failure

netdevsim: drop PSP ext ref on forward failure

nsim_do_psp() takes an extra reference to the PSP skb extension so the
extension survives __dev_forward_skb(). That forward path scrubs the skb
and drops attached skb extensions before nsim_psp_handle_ext() can
reattach the PSP metadata.

If __dev_forward_skb() fails in nsim_forward_skb(), the function returns
before nsim_psp_handle_ext() can attach that extension to the skb, leaving
the extra reference leaked.

Drop the saved PSP extension reference before returning from the
forward-failure path. Guard the put because plain or non-decapsulated
traffic can also fail forwarding without ever taking the extra PSP
reference.

Fixes: f857478d6206 ("netdevsim: a basic test PSP implementation")
Signed-off-by: Wesley Atwell <atwellwea@gmail.com>
Reviewed-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20260317061431.1482716-1-atwellwea@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>