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

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

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>
Mainline v4.7 commit f0fe970df3838c202ef6c07a4c2b36838ef0a88b
Replaces UEK4 commit e06914f2e9ac6b3f19d4461cb24b401f77ce4f17
which was reverted by UEK4 commit b1660e855b21.

Orabug: 24971905
CVE: CVE-2016-1583
Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com