ppp: release rtnl mutex when interface creation fails

Add missing rtnl_unlock() in the error path of ppp_create_interface().

Fixes: 58a89eca

 ("ppp: fix lockdep splat in ppp_dev_uninit()")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>