ext4: start to support iopoll method

Since commit "b1b4705d ext4: introduce direct I/O read using
iomap infrastructure", we can easily make ext4 support iopoll
method, just use iomap_dio_iopoll().
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20200207120758.2411-1-xiaoguang.wang@linux.alibaba.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/file.c

@@ -872,6 +872,7 @@ const struct file_operations ext4_file_operations = {
 	.llseek		= ext4_llseek,
 	.read_iter	= ext4_file_read_iter,
 	.write_iter	= ext4_file_write_iter,
+	.iopoll		= iomap_dio_iopoll,
 	.unlocked_ioctl = ext4_ioctl,
 #ifdef CONFIG_COMPAT
 	.compat_ioctl	= ext4_compat_ioctl,