gfs2: Align read and write chunks to the page cache

Align the chunks that reads and writes are carried out in to the page
cache rather than the user buffers.  This will be more efficient in
general, especially for allocating writes.  Optimizing the case that the
user buffer is gfs2 backed isn't very useful; we only need to make sure
we won't deadlock.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>