freezer: PF_FREEZER_NOSIG should be cleared along with PF_NOFREEZE

This patch is only for pre-v3.3 stable trees which backported
b40a7959 "freezer: exec should clear PF_NOFREEZE along with PF_KTHREAD".
v3.3+ doesn't need this fix.

b40a7959 is the trivial bugfix, but unfortunately I forgot that
until 34b087e4 "freezer: kill unused set_freezable_with_signal()"
there were another only-for-kernel-threads flag, PF_FREEZER_NOSIG,
which should be cleared as well.

See https://bugs.launchpad.net/ubuntu/+source/v86d/+bug/1080530
The freezer fails because it expects that a PF_FREEZER_NOSIG task
doesn't need a signal. Before b40a7959 it wrongly succeeds leaving
the PF_NOFREEZE | PF_FREEZER_NOSIG task unfrozen.
Reported-and-tested-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[bwh: Don't touch PF_FORKNOEXEC; it's cleared elsewhere]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/exec.c

@@ -1095,7 +1095,8 @@ int flush_old_exec(struct linux_binprm * bprm)
 	bprm->mm = NULL;		/* We're using it now */
 
 	set_fs(USER_DS);
-	current->flags &= ~(PF_RANDOMIZE | PF_KTHREAD | PF_NOFREEZE);
+	current->flags &=
+		~(PF_RANDOMIZE | PF_KTHREAD | PF_NOFREEZE | PF_FREEZER_NOSIG);
 	flush_thread();
 	current->personality &= ~bprm->per_clear;