GFS2: No need to invalidate pages for a dio read

We recently fixed the writeback of pages prior to performing
direct i/o, however the initial fix was perhaps a bit heavy
handed. There is no need to invalidate pages if the direct i/o
is only a read, since they will be identical to what has been
flushed to disk anyway.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>