f2fs: cover update_free_nid_bitmap with nid_list_lock

free_nid_bitmap and free_nid_count in update_free_nid_bitmap should be
updated atomically, use nid_list_lock cover them to avoid race in
concurrent scenario.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>