coredump: use required compat_nr_pages as fallback for nr_pages

[alam0rt]

Noticed that coredump was blowing up when constructing a coredump from a checkpoint (arm64).

Traceback (most recent call last):
  File "/home/ubuntu_ssm_sudo/checkpoint/checkpoint/../../criu/coredump/coredump", line 55, in <module>
    main()
  File "/home/ubuntu_ssm_sudo/checkpoint/checkpoint/../../criu/coredump/coredump", line 47, in main
    coredump(opts)
  File "/home/ubuntu_ssm_sudo/checkpoint/checkpoint/../../criu/coredump/coredump", line 14, in coredump
    cores = generator(os.path.realpath(opts['in']))
  File "/home/ubuntu_ssm_sudo/criu/coredump/criu_coredump/coredump.py", line 192, in __call__
    self.coredumps[pid] = self._gen_coredump(pid)
  File "/home/ubuntu_ssm_sudo/criu/coredump/criu_coredump/coredump.py", line 214, in _gen_coredump
    cd.vmas = self._gen_vmas(pid)
  File "/home/ubuntu_ssm_sudo/criu/coredump/criu_coredump/coredump.py", line 991, in _gen_vmas
    v.data = self._gen_mem_chunk(pid, vma, v.filesz)
  File "/home/ubuntu_ssm_sudo/criu/coredump/criu_coredump/coredump.py", line 878, in _gen_mem_chunk
    page_mem = self._get_page(pid, page_no)
  File "/home/ubuntu_ssm_sudo/criu/coredump/criu_coredump/coredump.py", line 797, in _get_page
    for i in range(m["nr_pages"]):
KeyError: 'nr_pages'

Since the pagemap contained compat_nr_pages instead of nr_pages, I've added a fallback to use that instead.

E.g., dict contained the below

{'vaddr': 281474652319744, 'compat_nr_pages': 2, 'flags': 6}

[avagin]

LGTM: Please add the "coredump: " prefix to the subject.

[rst0git]

LGTM