dm-exception-store.c: Fix error cleanup in dm_create_persistent().
This was originally found by chrisw during code review.

From: Dave Olien <dmo@osdl.org>
Signed-off-by: Kevin Corry <kevcorry@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>