Revert "debugfs: inode: debugfs_create_dir uses mode permission from parent"

This reverts commit 95cde3c5.

The commit had good intentions, but it breaks kvm-tool and qemu-kvm.

With it in place, "lkvm run" just fails with

  Error: KVM_CREATE_VM ioctl
  Warning: Failed init: kvm__init

which isn't a wonderful error message, but bisection pinpointed the
problematic commit.

The problem is almost certainly due to the special kvm debugfs entries
created dynamically by kvm under /sys/kernel/debug/kvm/.  See
kvm_create_vm_debugfs()

Bisected-and-reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>