commit dd4c1b7d upstream.

If the number_of_areas argument was zero the kernel would crash on
div-by-zero.  Add better input validation.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>