Revert "arp: Prevent overflow in arp_req_get()."

This reverts commit f119f232 which is
commit a7d60277 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I97be23b4073709843571e5b5d05903fda4121870
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>