cifs: invalidate cache when we truncate a file

RHBZ: 1566345

When truncating a file we always do this synchronously to the server.
Thus we need to make sure that the cached inode metadata is
marked as stale so that on next getattr we will refresh the metadata.
In this particular bug we want to ensure that both ctime and mtime
are updated and become visible to the application after a truncate.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reported-by: Xiaoli Feng <xifeng@redhat.com>