ovl: opaque cleanup

oe->opaque is set for

 a) whiteouts
 b) directories having the "trusted.overlay.opaque" xattr

Case b can be simplified, since setting the xattr always implies setting
oe->opaque.  Also once set, the opaque flag is never cleared.

Don't need to set opaque flag for non-directories.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>