criu: fix filemap open permissions

An mmaped file is opened O_RDONLY or O_RDWR depending on the permissions
on the first vma dump_task_mm() encounters mapping that file. This
causes two problems:

1. If a file has multiple MAP_SHARED mappings, some of which are
   read-only and some of which are read-write, and the first encountered
   mapping happens to be read-only, the file will be opened O_RDONLY
   during restore, and mmap(PROT_WRITE) will fail with EACCES, causing
   the restore to fail.

2. If a file is opened read-write and mapped read-only, it will be
   opened O_RDONLY during restore, so restore will succeed, but
   mprotect(PROT_WRITE) on the read-only mapping after restore will
   fail.

To fix both of these, record open flags per-vma based on the presence of
VM_MAYWRITE in smaps.

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