net, uapi: remove inclusion of arpa/inet.h

In include/uapi/linux/tipc_config.h, there's a comment that it includes
arpa/inet.h for ntohs; but ntohs is not defined in any UAPI header. For
now, reuse the definitions from include/linux/byteorder/generic.h, since
the various conversion functions do exist in UAPI headers:
include/uapi/linux/byteorder/big_endian.h
include/uapi/linux/byteorder/little_endian.h

We would like to get to the point where we can build UAPI header tests
with -nostdinc, meaning that kernel UAPI headers should not have a
circular dependency on libc headers.

Link: https://android-review.googlesource.com/c/platform/bionic/+/2048127


Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>