[IPV4]: Make struct in_addr::s_addr __be32

There will be relatively small increase in sparse endian warnings, but
this (and sin_port) patch is a first step to make networking code
endian clean.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>