smb3: fix performance regression with setting mtime

There are cases when we don't want to send the SMB2 flush operation
(e.g. when user specifies mount parm "nostrictsync") and it can be
a very expensive operation on the server.  In most cases in order
to set mtime, we simply need to flush (write) the dirtry pages from
the client and send the writes to the server not also send a flush
protocol operation to the server.

Fixes: aa081859 ("cifs: flush before set-info if we have writeable handles")
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>