Ocfs2: Fix a regression bug from mainline commit(6b933c8e).

The patch is to fix the regression bug brought from commit 6b933c8e...( 'ocfs2:
Avoid direct write if we fall back to buffered I/O'):

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



The commit 6b933c8e 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>