[GFS2] Update truncate function (shrinking partial blocks)

Update the function in GFS2 which deals with truncation of
partial blocks. Some of the code is "borrowed" from ext3
since it appears to give a good model of how to do this
operation. The function is renamed gfs2_block_truncate_page
accordingly.

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