proc: prevent stacking filesystems on top

This prevents stacking filesystems (ecryptfs and overlayfs) from using
procfs as lower filesystem.  There is too much magic going on inside
procfs, and there is no good reason to stack stuff on top of procfs.

(For example, procfs does access checks in VFS open handlers, and
ecryptfs by design calls open handlers from a kernel thread that doesn't
drop privileges or so.)

Signed-off-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
Git-commit: e54ad7f1ee263ffa5a2de9c609d58dfa27b21cd9
(cherry picked from commit e54ad7f1ee263ffa5a2de9c609d58dfa27b21cd9)
Change-Id: I1bf47b15e8201d3a049a04e1b054c664d9be9bea