freezer: make exiting tasks properly unfreezable

freezer: make exiting tasks properly unfreezable

There's no point in freezing an exiting task.  The current code
seemingly tries that by calling clear_freeze_flag() from exit_mm() but
it's racy as freeze might happen afterwards.

This patch removes the racy clear_freeze_flag() makes do_exit() set
PF_NOFREEZE after PTRACE_EVENT_EXIT, after which freezing doesn't make
sense.

Signed-off-by: Tejun Heo <tj@kernel.org>