ocfs2: Avoid direct write if we fall back to buffered I/O

when we fall back to buffered write from direct write, we call
__generic_file_aio_write() but that will end up doing direct write
even we are only prepared to do buffered write because the file
has the O_DIRECT flag set. This is a fix for
https://bugzilla.novell.com/show_bug.cgi?id=591039


revised with Joel's comments.

Signed-off-by: Li Dongyang <lidongyang@novell.com>
Acked-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>