UBI: block: Set disk_capacity out of the mutex

There's no need to set the disk capacity with the mutex held, so this
commit takes the variable setting out of the mutex. This simplifies
the disk capacity fix for very large volumes in a follow up commit.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>