netlink: allow empty nested attributes

validate_nla() currently doesn't allow empty nested attributes. This
makes userspace code unnecessarily complicated when starting and ending
the nested attribute is done by generic upper level code and the inner
attributes are dumped by a module.

Add a special case to accept empty nested attributes. When the nested
attribute is non empty, the same checks as before are performed.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>