Assemble: don't use O_EXCL until we have checked device content.

Assemble: don't use O_EXCL until we have checked device content.

If we open with O_EXCL before checking that the device is one that
we really want, then that could cause some other process to think
the device is busy when it isn't really.

This particularly affects running "mdadm -A devname" in parallel for
different arrays.  One might be looking at a device that it won't
end up using while another trys and fails to look at a device that
it needs.

So delay the O_EXCL until after all identity checks.

Multiple "mdadm -As" will still have races, but that is fundamentally
racy anyway.

Signed-off-by: NeilBrown <neilb@suse.de>