UPSTREAM: Revert "ecryptfs: forbid opening files without mmap handler"

(cherry picked from commit 78c4e172412de5d0456dc00d2b34050aa0b683b5)

This reverts upstream commit 2f36db71009304b3f0b95afacd8eba1f9f046b87.

It fixed a local root exploit but also introduced a dependency on
the lower file system implementing an mmap operation just to open a file,
which is a bit of a heavy hammer.  The right fix is to have mmap depend
on the existence of the mmap handler instead.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>

Fixes: Change-Id I0be77c7f8bd3046bc34cd87ef577529792d479bc
       ("UPSTREAM: ecryptfs: forbid opening files without mmap handler")
Change-Id: Ib9bc87099f7f89e4e12dbc1a79e884dcadb1befb
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>