Fix race between --create and --incremental

Fix race between --create and --incremental

This modifies locking in Create to eliminate a situation where
--incremental can assemble a device between write_init_super() and
add_disk(), which causes Create to fail.

It sporadically occurs e.g. when metadata is written on a device,
causing an udev change event which triggers mdadm --incremental.

Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>