ubi: fastmap: Use kmem_cache_free to deallocate memory

Memory allocated by kmem_cache_alloc() should not be deallocated with
kfree(). Use kmem_cache_free() instead.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>