neighbor: Remove externally learned entries from gc_list

Externally learned entries are similar to PERMANENT entries in the
sense they are managed by userspace and can not be garbage collected.
As such remove them from the gc_list, remove the flags check from
neigh_forced_gc and skip threshold checks in neigh_alloc. As with
PERMANENT entries, this allows unlimited number of NTF_EXT_LEARNED
entries.

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