net: Rename net/nexthop.h net/rtnh.h

The header contains rtnh_ macros so rename the file accordingly.
Allows a later patch to use the nexthop.h name for the new
nexthop code.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>