[GFS2] Fix bug in writepage()

As pointed out by Wendy Cheng, the logic in GFS2's writepage() function
wasn't quite right with respect to invalidating pages when a file has been
truncated. This patch fixes that.

CC: Wendy Cheng <wcheng@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>