UBI: Update comments to reflect UBI_METAONLY flag

This patch trivially updates code comments to reflect the addition of the
UBI_METAONLY flag - as discussed https://lkml.org/lkml/2014/10/29/764



Cc: Richard Weinberger <richard@nod.at>
Cc: trivial@kernel.org
Signed-off-by: Andrew Murray <amurray@embedded-bits.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>