xen-netfront: Fix mismatched rtnl_unlock

[ Upstream commit cb257783c2927b73614b20f915a91ff78aa6f3e8 ]

Fixes: f599c64fdf7d ("xen-netfront: Fix race between device setup and open")
Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>