f2fs: no need to lock for update_inode_page all the time

As comment says, we don't need to call f2fs_lock_op in write_inode to prevent
from producing dirty node pages all the time.
That happens only when there is not enough free sections and we can avoid that
by calling balance_fs in prior to that.
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/inode.c

@@ -296,16 +296,12 @@ int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
 		return 0;
 
 	/*
-	 * We need to lock here to prevent from producing dirty node pages
+	 * We need to balance fs here to prevent from producing dirty node pages
 	 * during the urgent cleaning time when runing out of free sections.
 	 */
-	f2fs_lock_op(sbi);
 	update_inode_page(inode);
-	f2fs_unlock_op(sbi);
-
-	if (wbc)
-		f2fs_balance_fs(sbi);
 
+	f2fs_balance_fs(sbi);
 	return 0;
 }