cifs: flush before set-info if we have writeable handles

cifs: flush before set-info if we have writeable handles

commit aa081859b10c5d8b19f5c525c78883a59d73c2b8 upstream.

Servers can defer destaging any data and updating the mtime until close().
This means that if we do a setinfo to modify the mtime while other handles
are open for write the server may overwrite our setinfo timestamps when
if flushes the file on close() of the writeable handle.

To solve this we add an explicit flush when the mtime is about to
be updated.

This fixes "cp -p" to preserve mtime when copying a file onto an SMB2 share.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>