vrf: move vrf_insert_slave so we can drop a goto label

We can simplify do_vrf_add_slave by moving vrf_insert_slave in the end
of the enslaving and thus eliminate an error goto label. It always
succeeds and isn't needed before that anyway.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>