f2fs: introduce get_dirty_dents for readability

The get_dirty_dents gives us the number of dirty dentry pages.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index 911b6f9..4c0e98d 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -619,7 +619,7 @@
 		return;
 
 	spin_lock(&sbi->dir_inode_lock);
-	if (atomic_read(&F2FS_I(inode)->dirty_dents)) {
+	if (get_dirty_dents(inode)) {
 		spin_unlock(&sbi->dir_inode_lock);
 		return;
 	}