Ocfs2: Fix a regression bug from mainline commit(54d1cb99adbab54817a09c06efe709166e4c...

Ocfs2: Fix a regression bug from mainline commit(54d1cb99adbab54817a09c06efe709166e4c7a7c).

The patch is to fix the regression bug brought from commit 54d1cb9...( 'ocfs2:
Avoid direct write if we fall back to buffered I/O'):

http://oss.oracle.com/bugzilla/show_bug.cgi?id=1285

The commit 54d1cb99adbab54817a09c06efe709166e4c7a7c changed __generic_file_aio_write
to generic_file_buffered_write, which didn't call filemap_{write,wait}_range to  flush
the pagecaches when we were falling O_DIRECT writes back to buffered ones. it did hurt
the O_DIRECT semantics somehow in extented odirect writes.

This patch tries to guarantee O_DIRECT writes of 'fall back to buffered' to be correctly
flushed.

Signed-off-by: Tristan Ye <tristan.ye@oracle.com>
Signed-off-by: Tao Ma <tao.ma@oracle.com>