security: change CR_FD_PERM from rw-rw-r-- to rw-r--r--

This makes only root to be able to modify images by default.
When using criu with suid bit set, group of the images is set
to user group, which is not safe, considering current CR_FD_PERM.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>