IB/ipoib: Fix memory leak in create child syscall

The flow of creating a new child goes through ipoib_vlan_add
which allocates a new interface and checks the rtnl_lock.

If the lock is taken, restart_syscall will be called to restart
the system call again. In this case we are not releasing the
already allocated interface, causing a leak.

Fixes: 9baa0b03 ("IB/ipoib: Add rtnl_link_ops support")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>