md/linear: avoid possible oops and array stop

Since commit ef286f6fa673cd7fb367e1b145069d8dbfcc6081
it has been important that each personality clears
->private in the ->stop() function, or sets it to a
attribute group to be removed.
linear.c doesn't.  This can sometimes lead to an oops,
though it doesn't always.

Suitable for 2.6.33-stable and 2.6.34.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org