Freeing non-slab objects is bad and results in an oops.  Fix it.

Reported-and-tested-by: Andrew Price <andy@andrewprice.me.uk>
Cc: Theodore Tso <tytso@mit.edu>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>