dm: sysfs skip output when device is being destroyed

commit 4d89b7b4 upstream.

Do not process sysfs attributes when device is being destroyed.

Otherwise code can cause
  BUG_ON(test_bit(DMF_FREEING, &md->flags));
in dm_put() call.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>