l2tp: Fix the connect status check in pppol2tp_getname

The sk->sk_state is bits flag, so need use bit operation check
instead of value check.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Tested-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>