virtio: balloon: fix missing unlock on error in fill_balloon()

virtio: balloon: fix missing unlock on error in fill_balloon()

Add the missing unlock before return from function fill_balloon()
in the error handling case.

Introduced by 9864a8 ("virtio_balloon: introduce migration primitives to
balloon pages").

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>