UBI: block: Add support for the UBI_VOLUME_UPDATED notification

Static volumes can change its 'used_bytes' when they get updated,
and so the block interface must listen to the UBI_VOLUME_UPDATED
notification to resize the block device accordingly.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@vger.kernel.org # v3.15+