GFS2: Only increase rs_sizehint

If an application does a sequence of (1) big write, (2) little write
we don't necessarily want to reset the size hint based on the smaller
size. The fact that they did any big writes implies they may do more,
and therefore we should try to allocate bigger block reservations, even
if the last few were small writes. Therefore this patch changes function
gfs2_size_hint so that the size hint can only grow; it cannot shrink.
This is especially important where there are multiple writers.

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