netfilter: br_netfilter: remove unused conditional and dead code

The SKB_DROP_REASON_IP_INADDRERRORS drop reason is never returned from
any function, as such it cannot be returned from the ip_route_input call
tree. The 'reason != SKB_DROP_REASON_IP_INADDRERRORS' conditional is
thus always true.

Looking back at history, commit 50038bf3 ("net: ip: make
ip_route_input() return drop reasons") changed the ip_route_input
returned value check in br_nf_pre_routing_finish from -EHOSTUNREACH to
SKB_DROP_REASON_IP_INADDRERRORS. It turns out -EHOSTUNREACH could not be
returned either from the ip_route_input call tree and this since commit
251da413 ("ipv4: Cache ip_error() routes even when not
forwarding.").

Not a fix as this won't change the behavior. While at it use
kfree_skb_reason.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>