When send_cpu_listeners() finds the orphaned listener it marks it as
!valid and drops listeners->sem.  Before it takes this sem for writing,
s->pid can be reused and add_del_listener() can wrongly try to re-use
this entry.

Change add_del_listener() to check ->valid = T.
Signed-off-by: NOleg Nesterov <oleg@redhat.com>
Reviewed-by: NVasiliy Kulikov <segoon@openwall.com>
Acked-by: NBalbir Singh <bsingharora@gmail.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>