dm: fix idr leak on module removal

BugLink: http://bugs.launchpad.net/bugs/823296

commit d15b774c2920d55e3d58275c97fbe3adc3afde38 upstream.

Destroy _minor_idr when unloading the core dm module.  (Found by kmemleak.)

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>