Use after free from pid_nr_ns()

There is use after free reported due to group
leader task is already freed but other tasks are
still holding the group leader task address in
task->group_leader pointer.

pid_nr_ns+0x10/0x38
cgroup_pidlist_start+0x144/0x400
cgroup_seqfile_start+0x1c/0x24
kernfs_seq_start+0x54/0x90
seq_read+0x15c/0x3a8
kernfs_fop_read+0x38/0x160
__vfs_read+0x28/0xc8
vfs_read+0x84/0xfc

Change-Id: Ib6b3fc75bf0d24a04455bf81d54900c21c434958
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>