opts: Allo to specify the maximum size of ghost files

For example we hit a case where systemd carries journal
file with 4M in size.

https://jira.sw.ru/browse/PSBM-38571



Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>