mount: validate mounts only once on dump (v3)

mntinfo contains mounts from all namespaces, so we can validate it only
once after collecting mounts.

v2: add a fake comment about goto
v3: add a real comment about goto
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>