net-test: do not allow propagation of mount flags to parent namespace

on a cuttlefish x86_64 aosp3 device without this change:

  $ adbz shell cat /proc/mounts | egrep ' /(|proc|sys) '
  /dev/block/dm-2 / ext4 ro,seclabel,noatime,errors=panic 0 0
  proc /proc proc rw,relatime,gid=3009,hidepid=2 0 0
  sysfs /sys sysfs rw,seclabel,relatime 0 0

  $ atest vts_kernel_net_tests
  ...

  $ adbz shell cat /proc/mounts | egrep ' /(|proc|sys) '
  /dev/block/dm-2 / ext4 ro,seclabel,noatime,errors=panic 0 0
  proc /proc proc rw,nosuid,nodev,noexec,relatime,gid=3009,hidepid=2 0 0
  sysfs /sys sysfs rw,seclabel,nosuid,nodev,noexec,relatime 0 0

with this change there is no longer a difference.

(This explains why I saw this mount syscall while strace'ing unshare)

Bug: 149894399
Test: ran 'atest vts_kernel_net_tests'
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ia64be091b58f97fdf7e0c24ca6599b1aecd3bcdf
Merged-In: Ia64be091b58f97fdf7e0c24ca6599b1aecd3bcdf
diff --git a/net/test/namespace.py b/net/test/namespace.py
index 0045588..986fb59 100644
--- a/net/test/namespace.py
+++ b/net/test/namespace.py
@@ -132,6 +132,7 @@
 
   try:
     # DumpMounts('Before:')
+    Mount('none', '/', None, MS_REC|MS_PRIVATE)
     ReMountProc()
     ReMountSys()
     # DumpMounts('After:')