ecryptfs: forbid opening files without mmap handler

ecryptfs: forbid opening files without mmap handler

Orabug: 24971905
CVE: CVE-2016-1583

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>
(cherry picked from commit 2f36db71009304b3f0b95afacd8eba1f9f046b87)
Signed-off-by: Brian Maly <brian.maly@oracle.com>