UBI_METAONLY is a new open mode for UBI volumes, it indicates
that only meta data is being changed.
Meta data in terms of UBI volumes means data which is stored in the
UBI volume table but not on the volume itself.
While it does not interfere with UBI_READONLY and UBI_READWRITE
it is not allowed to use UBI_METAONLY together with UBI_EXCLUSIVE.

Cc: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: Andrew Murray <amurray@embedded-bits.co.uk>
Signed-off-by: NRichard Weinberger <richard@nod.at>
Tested-by: NGuido Martínez <guido@vanguardiasur.com.ar>
Reviewed-by: NGuido Martínez <guido@vanguardiasur.com.ar>
Tested-by: NChristoph Fritz <chf.fritz@googlemail.com>
Tested-by: NAndrew Murray <amurray@embedded-bits.co.uk>