ocfs2: prepare some interfaces used in append direct io

ocfs2: prepare some interfaces used in append direct io

Currently in case of append O_DIRECT write (block not allocated yet),
ocfs2 will fall back to buffered I/O.  This has some disadvantages.
Firstly, it is not the behavior as expected.  Secondly, it will consume
huge page cache, e.g.  in mass backup scenario.  Thirdly, modern
filesystems such as ext4 support this feature.

In this patch set, the direct I/O write doesn't fallback to buffer I/O
write any more because the allocate blocks are enabled in direct I/O now.

This patch (of 9):

Prepare some interfaces which will be used in append O_DIRECT write.

Signed-off-by: Joseph Qi <[email protected]>
Cc: Weiwei Wang <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Xuejiufei <[email protected]>
Cc: Junxiao Bi <[email protected]>
Cc: alex chen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>