dax: Remove pointless writeback from dax_do_io()

dax_do_io() is calling filemap_write_and_wait() if DIO_LOCKING flags is
set. Presumably this was copied over from direct IO code. However DAX
inodes have no pagecache pages to write so the call is pointless. Remove
it.
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>

fs/dax.c

@@ -268,15 +268,8 @@ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode,
 	memset(&bh, 0, sizeof(bh));
 	bh.b_bdev = inode->i_sb->s_bdev;
 
-	if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ) {
-		struct address_space *mapping = inode->i_mapping;
+	if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ)
 		inode_lock(inode);
-		retval = filemap_write_and_wait_range(mapping, pos, end - 1);
-		if (retval) {
-			inode_unlock(inode);
-			goto out;
-		}
-	}
 
 	/* Protects against truncate */
 	if (!(flags & DIO_SKIP_DIO_COUNT))
@@ -297,7 +290,6 @@ ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode,
 
 	if (!(flags & DIO_SKIP_DIO_COUNT))
 		inode_dio_end(inode);
- out:
 	return retval;
 }
 EXPORT_SYMBOL_GPL(dax_do_io);