UBUNTU: SAUCE: overlayfs: fix incorrect mnt_id of files opened from map_files

UBUNTU: SAUCE: overlayfs: fix incorrect mnt_id of files opened from map_files

BugLink: https://bugs.launchpad.net/bugs/1967924
BugLink: https://bugs.launchpad.net/bugs/1857257
The hack was introduced in ("UBUNTU: SAUCE: overlayfs: allow with
shiftfs as underlay") and it broke checkpoint/restore of docker
contains:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1857257

The following script can be used to trigger the issue:
  #!/bin/bash

  cat > test.py << EOF
  import sys

  f = open("/proc/self/maps")

  for l in f.readlines():
    if "python" not in l:
      continue
    print(l)
    s = l.split()
    start, end = s[0].split("-")
    fname = s[-1]
    print(start, end, fname)
    break
  else:
    sys.exit(1)

  test_file1 = open(fname)
  test_file2 = open("/proc/self/map_files/%s-%s" % (start, end))

  fdinfo1 = open("/proc/self/fdinfo/%d" % test_file1.fileno()).read()
  fdinfo2 = open("/proc/self/fdinfo/%d" % test_file2.fileno()).read()

  if fdinfo1 != fdinfo2:
    print("FAIL")
    print(test_file1)
    print(fdinfo1)
    print(test_file2)
    print(fdinfo2)
    sys.exit(1)
  print("PASS")
  EOF
  sudo docker run -it --privileged --rm -v `pwd`:/mnt python python /mnt/test.py

Thanks to Andrei Vagin for the reproducer and investigation of this problem.

Cc: Andrei Vagin <avagin@gmail.com>
Cc: Adrian Reber <areber@redhat.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Stefan Bader <stefan.bader@canonical.com>
Cc: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Fixes: d24b8a5 ("UBUNTU: SAUCE: overlayfs: allow with shiftfs as underlay")
Signed-off-by: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>
[small refactoring to add the dependency of AUFS for vma->vm_prfile]
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>