UPSTREAM: ecryptfs: forbid opening files without mmap handler

(cherry picked from commit 2f36db71)

This prevents users from triggering a stack overflow through a recursive
invocation of pagefault handling that involves mapping procfs files into
virtual memory.

Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Change-Id: I0be77c7f8bd3046bc34cd87ef577529792d479bc
Bug: 29444228