mdopen: open md devices O_RDONLY

mdopen: open md devices O_RDONLY

There is no need to request write access when opening
the md device, as we never write to it, and none of the
ioctls we use require write access.

If we do open with write access, then when we close, udev notices that
the device was closed after being open for write access, and it
generates a CHANGE event.

This is generally unwanted, and particularly problematic when mdadm is
trying to --stop the array, as the CHANGE event can cause the array to
be re-opened before it completely closed, which results in a new mddev
being allocated.

So just use O_RDONLY instead of O_RDWR.

Reported-by: Marc Smith <marc.smith@mcc.edu>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>