[CIFS] O_DIRECT opens should work on directio mounts

Opens on current cifs/smb2/smb3 mounts with O_DIRECT flag fail
even when caching is disabled on the mount.  This was
reported by those running SMB2 benchmarks who need to
be able to pass O_DIRECT on many of their open calls to
reduce caching effects, but would also be needed by other
applications.

When mounting with forcedirectio ("cache=none") cifs and smb2/smb3
do not go through the page cache and thus opens with O_DIRECT flag
should work (when posix extensions are negotiated we even are
able to send the flag to the server). This patch fixes that
in a simple way.

The 9P client has a similar situation (caching is often disabled)
and takes the same approach to O_DIRECT support ie works if caching
disabled, but if client caching enabled it fails with EINVAL.

A followon idea for a future patch as Pavel noted, could
be that files opened with O_DIRECT could cause us to change
inode->i_fop on the fly from

cifs_file_strict_ops

to

cifs_file_direct_ops

which would allow us to support this on non-forcedirectio mounts
(cache=strict and cache=loose) as well.
Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/file.c

@@ -3663,6 +3663,27 @@ void cifs_oplock_break(struct work_struct *work)
 	}
 }
 
+/*
+ * The presence of cifs_direct_io() in the address space ops vector
+ * allowes open() O_DIRECT flags which would have failed otherwise.
+ *
+ * In the non-cached mode (mount with cache=none), we shunt off direct read and write requests
+ * so this method should never be called.
+ *
+ * Direct IO is not yet supported in the cached mode. 
+ */
+static ssize_t
+cifs_direct_io(int rw, struct kiocb *iocb, const struct iovec *iov,
+               loff_t pos, unsigned long nr_segs)
+{
+        /*
+         * FIXME
+         * Eventually need to support direct IO for non forcedirectio mounts
+         */
+        return -EINVAL;
+}
+
+
 const struct address_space_operations cifs_addr_ops = {
 	.readpage = cifs_readpage,
 	.readpages = cifs_readpages,
@@ -3672,6 +3693,7 @@ const struct address_space_operations cifs_addr_ops = {
 	.write_end = cifs_write_end,
 	.set_page_dirty = __set_page_dirty_nobuffers,
 	.releasepage = cifs_release_page,
+	.direct_IO = cifs_direct_io,
 	.invalidatepage = cifs_invalidate_page,
 	.launder_page = cifs_launder_page,
 };