net/checksum.h pulls in linux/uaccess.h which is large.

In the x86 header the include seems to not be needed at all.
ARM on the other hand does not include uaccess.h, even tho
it calls access_ok().

In the generic implementation guard the include of linux/uaccess.h
with the same condition as the code that needs it.

With this change pre-processed net/checksum.h shrinks on x86
from 30616 lines to just 1193.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>