Revert "ecryptfs: forbid opening files without mmap handler"

Revert "ecryptfs: forbid opening files without mmap handler"

This reverts commit 43c84d81bfb6740073147d409dc980b1697fb1ef.

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>