neighbor: Move neigh_update_ext_learned to core file

neigh_update_ext_learned has one caller in neighbour.c so does not need
to be defined in the header. Move it and in the process remove the
intialization of ndm_flags and just set it based on the flags check.

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