jffs2: move jffs2_gcd_mtd threads to the new kthread API

Move the jffs2 garbage collecting thread to the new kthread API.

Signed-off-by: Gerard Lledo <gerard.lledo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>