net: Remove __KERNEL__ cpp checks from include/net

These header files are never installed to user consumption, so any
__KERNEL__ cpp checks are superfluous.

Projects should also not copy these files into their userland utility
sources and try to use them there.  If they insist on doing so, the
onus is on them to sanitize the headers as needed.

Signed-off-by: David S. Miller <davem@davemloft.net>