parallels: improve readability of allocate_clusters

parallels: improve readability of allocate_clusters

Replace 'space' representing the amount of data to preallocate with
'bytes'.

Rationale:
* 'space' at each place is converted to bytes
* the unit is more close to the variable name

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>