ecryptfs: fixed msync to flush data

ecryptfs: fixed msync to flush data

commit c15cddd900e867c5adfb3c79596479dc5975f743 upstream.

When msync is called on a memory mapped file, that
data is not flushed to the disk.

In Linux, msync calls fsync for the file. For ecryptfs,
fsync just calls the lower level file system's fsync.
Changed the ecryptfs fsync code to call filemap_write_and_wait
before calling the lower level fsync.

Addresses the problem described in http://crbug.com/239536

Signed-off-by: Paul Taysom <taysom@chromium.org>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>