mount: save local mount point paths on restore

On restore we add a temporary root to a mount point path. It's convinient
for restoring mount namespaces, but real paths are used for restoring
link-remap files.

v2: replace the offset field on a char * field

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>