files-reg: fix "criu: fix filemap open permissions"

Fixes two issues with efe594f8 "criu: fix filemap open permissions":

- Permissions on files with both open file descriptors and mappings.

- Restore compatibility with dumps created by previous versions of criu.

Signed-off-by: Jamie Liu <jamieliu@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>