vmas: Don't call stat on vm file twice

When parsing mappings in proc, we fstat vm file, later,
when dumping it, we stat it again to fill fd_parms.
The 2nd stat is not required, we can keep the stat in
vma_area.

This removed 35% of all stat calls on dump of basic container.

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