mdopen: use parameters/new_array to create arrays whenever possible.

mdopen: use parameters/new_array to create arrays whenever possible.

In a sufficiently recent kernel, an md%d array can be
created by writing to .../parameters/new_array.
If mdadm does this consistently, then another new
feature, disabling create_on_open, can be enabled.
This avoids races on shutdown.

An added benefit of using new_array (where available)
is that it allows md arrays with numbers larger than 511
(e.g. md999) to be created.  The old create_on_open
mechanism doesn't support such devices since
Commit: af5628f05db6 ("md: disable probing for md devices 512 and over.")
in Linux 3.17.

After a few more mdadm releases it would be good to
have mdadm disable create_on_open automatically.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>