ipv4: route: fix drop reason being overridden in ip_route_input_slow

ipv4: route: fix drop reason being overridden in ip_route_input_slow

When jumping to 'martian_destination' a drop reason is always set but
that label falls-through the 'e_nobufs' one, overriding the value.

The behavior was introduced by the mentioned commit. The logic went
from,

	goto martian_destination;
	...
  martian_destination:
	...
  e_inval:
	err = -EINVAL;
	goto out;
  e_nobufs:
	err = -ENOBUFS;
	goto out;

to,

	reason = ...;
	goto martian_destination;
	...
  martian_destination:
	...
  e_nobufs:
	reason = SKB_DROP_REASON_NOMEM;
	goto out;

A 'goto out' is clearly missing now after 'martian_destination' to avoid
overriding the drop reason.

Fixes: 5b92112acd8e ("net: ip: make ip_route_input_slow() return drop reasons")
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Cc: Menglong Dong <menglong8.dong@gmail.com>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250108165725.404564-1-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>