f2fs: introduce scan_curseg_cache for cleanup

Commit 4ac91242 ("f2fs: introduce free nid bitmap") copied codes
from __build_free_nids() into scan_free_nid_bits(), they are redundant,
introduce one common function scan_curseg_cache for cleanup.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>