misc/tst-clone3: Fix waiting for exited thread.

misc/tst-clone3: Fix waiting for exited thread.

From time to time the test misc/tst-clone3 fails with a timeout.
Then futex_wait is blocking.  Usually ctid should be set to zero
due to CLONE_CHILD_CLEARTID and the futex should be waken up.
But the fail occures if the thread has already exited before
ctid is set to the return value of clone().  Then futex_wait() will
block as there will be nobody who wakes the futex up again.

This patch initializes ctid to a known value before calling clone
and the kernel is the only one who updates the value to zero after clone.
If futex_wait is called then it is either waked up due to the exited thread
or the futex syscall fails as *ctid_ptr is already zero instead of the
specified value 1.

ChangeLog:

	* sysdeps/unix/sysv/linux/tst-clone3.c (do_test):
	Initialize ctid with a known value and remove update of ctid
	after clone.
	(wait_tid): Adjust arguments and call futex_wait with ctid_val
	as assumed current value of ctid_ptr.