zram: avoid double free in function zram_bvec_write()

commit 65c484609a3b25c35e4edcd5f2c38f98f5226093 upstream.

When doing a patial write and the whole page is filled with zero,
zram_bvec_write() will free uncmem twice.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>