l2tp: Correctly return -EBADF from pppol2tp_getname.

If 'tunnel' is NULL we should return -EBADF but the 'end_put_sess' path
unconditionally sets 'error' back to zero. Rework the error path so it
more closely matches pppol2tp_sendmsg.

Fixes: fd558d18 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>