md/raid5: don't test ->writes_pending in raid5_remove_disk

This test on ->writes_pending cannot be safe as the counter
can be incremented at any moment and cannot be locked against.

Change it to test conf->active_stripes, which at least
can be locked against.  More changes are still needed.

A future patch will change ->writes_pending, and testing it here will
be very inconvenient.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>