ocfs2: Return -EINVAL when a device is not ocfs2.

In case of non-modular kernels the root filesystem is mounted by trying
several filesystems. If ocfs2 was tried before the actual filesystem
type, the mount would fail because ocfs2_sb_probe() returns -EAGAIN
instead of -EINVAL.  ocfs2 will now return -EINVAL properly.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Reported-by: Laszlo Attila Toth <panther@balabit.hu>