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

This reverts commit dbc9b22d 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: I3fb6c3a65b627f454c746e3b1b1dd236947c6035
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>