parallels: Add checking and repairing duplicate offsets in BAT

parallels: Add checking and repairing duplicate offsets in BAT

Cluster offsets must be unique among all the BAT entries. Find duplicate
offsets in the BAT and fix it by copying the content of the relevant
cluster to a newly allocated cluster and set the new cluster offset to the
duplicated entry.

Add host_cluster_index() helper to deduplicate the code.

When new clusters are allocated, the file size increases by 128 Mb. Call
parallels_check_leak() to fix this leak.

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