ecryptfs: forbid opening files without mmap handler

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

Change-Id: I91f0a2b4fecaa7deaef9d5b062dca1b88a083703
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>
Git-repo: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Git-commit: 2f36db71009304b3f0b95afacd8eba1f9f046b87
Signed-off-by: Ravi Kumar Siddojigari <rsiddoji@codeaurora.org>