nhrpd: Create route to private spoke-spoke network correctly

nhrpd: Create route to private spoke-spoke network correctly

Currently when the first traffic to a private network causes a shortcut, an
on-link route to the private network is created on the gre interface, along
with the cache entry.

When connecting to a second IP in the same network, the kernel tries to resolve
the public IP for this private network via query to NHRP. nhrpd sees no entry
in the cache, so the packet is dropped.

The fix to this solution can be instead of creating an on-link route, create an
off-link route to private network, with the next-hop being the remote tunnel's
gre IP address.

Signed-off-by: Reuben Dowle <reuben.dowle@4rf.com>