ocfs2: Pagecache usage optimization on ocfs2

A page can have multiple buffers and even if a page is not uptodate, some buffers
can be uptodate on pagesize != blocksize environment.
This aops checks that all buffers which correspond to a part of a file
that we want to read are uptodate. If so, we do not have to issue actual
read IO to HDD even if a page is not uptodate because the portion we
want to read are uptodate.
"block_is_partially_uptodate" function is already used by ext2/3/4.
With the following patch random read/write mixed workloads or random read after
random write workloads can be optimized and we can get performance improvement.
Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>

fs/ocfs2/aops.c

@@ -1967,4 +1967,5 @@ const struct address_space_operations ocfs2_aops = {
 	.invalidatepage		= ocfs2_invalidatepage,
 	.releasepage		= ocfs2_releasepage,
 	.migratepage		= buffer_migrate_page,
+	.is_partially_uptodate	= block_is_partially_uptodate,
 };