UBI: remove redundant mutex

The @mult_mutex does not serve any purpose. We already have
@volumes_mutex and it is enough. The @volume mutex is pushed
down to the 'ubi_rename_volumes()', because we want first
to open all volumes in the exclusive mode, and then lock the
mutex, just like all other ioctl's (remove, re-size, etc) do.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>