files: link remap -- Fix potential buffer overrun

If the former file had a path long enough to be near
PATH_MAX limit, sprintf'ing link_remap.%d here might
overrun the limit. Use snprintf instead.

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