btrfs, nfs, apparmor: don't pull mnt_namespace.h for no reason...

it's not needed anymore; we used to, back when we had to do
mount_subtree() by hand, complete with put_mnt_ns() in it.
No more...  Apparmor didn't need it since the __d_path() fix.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>