mnt: fs_fully_visible test the proper mount for MNT_LOCKED

mnt: fs_fully_visible test the proper mount for MNT_LOCKED

commit d71ed6c930ac7d8f88f3cef6624a7e826392d61f upstream.

MNT_LOCKED implies on a child mount implies the child is locked to the
parent.  So while looping through the children the children should be
tested (not their parent).

Typically an unshare of a mount namespace locks all mounts together
making both the parent and the slave as locked but there are a few
corner cases where other things work.

Fixes: ceeb0e5d39fc ("vfs: Ignore unlocked mounts in fs_fully_visible")
Reported-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>