[IPV4]: Remove bogus ifdef mess in arp_process

[ Upstream commit: 3660019e ]

The #ifdef's in arp_process() were not only a mess, they were also wrong
in the CONFIG_NET_ETHERNET=n and (CONFIG_NETDEV_1000=y or
CONFIG_NETDEV_10000=y) cases.

Since they are not required this patch removes them.

Also removed are some #ifdef's around #include's that caused compile
errors after this change.
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>