vfork: kill PF_STARTING

Previously it was (ab)used by utrace.  Then it was wrongly used by the
scheduler code.

Currently it is not used, kill it before it finds the new erroneous user.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>