mount: add a comment to explain why do we need a clean mount
mount: add a comment to explain why do we need a clean mount

It's required when a target path is overmounted by one of child mounts.

Signed-off-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,KKoukiou/criu-remote,efiop/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,efiop/criu,eabatalov/criu,efiop/criu,KKoukiou/criu-remote,eabatalov/criu,eabatalov/criu,eabatalov/criu,efiop/criu,efiop/criu,efiop/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,eabatalov/criu