f2fs: use unlikely for release case

Since the variable release is only nonzero when another unlikely
case occurs, use unlikely() on it seems logical.

Signed-off-by: Fan li <fanofcode.li@samsung.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>