f2fs: enhance f2fs_is_checkpoint_ready()'s readability

This patch changes sematics of f2fs_is_checkpoint_ready()'s return
value as: return true when checkpoint is ready, other return false,
it can improve readability of below conditions.

f2fs_submit_page_write()
...
	if (is_sbi_flag_set(sbi, SBI_IS_SHUTDOWN) ||
				!f2fs_is_checkpoint_ready(sbi))
		__submit_merged_bio(io);

f2fs_balance_fs()
...
	if (!f2fs_is_checkpoint_ready(sbi))
		return;

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

fs/f2fs/xattr.c

@@ -732,9 +732,8 @@ int f2fs_setxattr(struct inode *inode, int index, const char *name,
 
 	if (unlikely(f2fs_cp_error(sbi)))
 		return -EIO;
-	err = f2fs_is_checkpoint_ready(sbi);
-	if (err)
-		return err;
+	if (!f2fs_is_checkpoint_ready(sbi))
+		return -ENOSPC;
 
 	err = dquot_initialize(inode);
 	if (err)