UPSTREAM: ecryptfs: don't allow mmap when the lower fs doesn't support it

(cherry picked from commit f0fe970d)

There are legitimate reasons to disallow mmap on certain files, notably
in sysfs or procfs.  We shouldn't emulate mmap support on file systems
that don't offer support natively.

CVE-2016-1583

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Cc: stable@vger.kernel.org
[tyhicks: clean up f_op check by using ecryptfs_file_to_lower()]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>

Change-Id: I66e3670771630a25b0608f10019d1584e9ce73a6
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>