f2fs: introduce release_discard_addr() for cleanup

Introduce release_discard_addr() to include common codes for cleanup.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Fengguang Wu: declare static function, reported by kbuild test robot]
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>