udf: add writepages support for udf

Use mpage_writepages() instead of multiple calls to udf_writepage()
to make performance higher.

*Write Speed with writepage() =
RecSize ReadSpeed WriteSpeed RanReadSpeed RanWriteSpeed
10485760 0.00MB/sec 8.56MB/sec 0.00MB/sec 8.20MB/sec
1048576 0.00MB/sec 8.57MB/sec 0.00MB/sec 6.42MB/sec
524288 0.00MB/sec 8.59MB/sec 0.00MB/sec 5.24MB/sec
262144 0.00MB/sec 8.59MB/sec 0.00MB/sec 4.17MB/sec
131072 0.00MB/sec 8.53MB/sec 0.00MB/sec 3.32MB/sec
65536 0.00MB/sec 8.49MB/sec 0.00MB/sec 2.31MB/sec

*Write Speed with writepages()
RecSize ReadSpeed WriteSpeed RanReadSpeed RanWriteSpeed
10485760 0.00MB/sec 9.88MB/sec 0.00MB/sec 9.60MB/sec
1048576 0.00MB/sec 9.95MB/sec 0.00MB/sec 7.52MB/sec
524288 0.00MB/sec 9.98MB/sec 0.00MB/sec 6.16MB/sec
262144 0.00MB/sec 9.90MB/sec 0.00MB/sec 4.98MB/sec
131072 0.00MB/sec 9.89MB/sec 0.00MB/sec 3.78MB/sec
65536 0.00MB/sec 9.81MB/sec 0.00MB/sec 2.50MB/sec

There is about 1.4MB/sec speed improvement over 8.5MB/sec,
which comes out around 16% improvement.

Signed-off-by: Namjae Jeon <[e-mail]>
Signed-off-by: Ashish Sangwan <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/udf/inode.c

···
 	return block_write_full_page(page, udf_get_block, wbc);
 }
 
+static int udf_writepages(struct address_space *mapping,
+			struct writeback_control *wbc)
+{
+	return mpage_writepages(mapping, wbc, udf_get_block);
+}
+
 static int udf_readpage(struct file *file, struct page *page)
 {
 	return mpage_readpage(page, udf_get_block);
···
 	.readpage	= udf_readpage,
 	.readpages	= udf_readpages,
 	.writepage	= udf_writepage,
+	.writepages	= udf_writepages,
 	.write_begin		= udf_write_begin,
 	.write_end		= generic_write_end,
 	.bmap		= udf_bmap,