restore: Parse /proc/self/maps for self mappings

On restore we only need to know currnet task mappings' start and end
to find where to put the restorer blob. And since the smaps file in
/proc/pid is up to 3 times slower, than the maps one, it makes
perfect sense just to parse the latter one.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>