[GFS2] Fix gfs2_block_truncate_page err return

Code segment inside gfs2_block_truncate_page() doesn't set the return
code correctly. This causes NFSD erroneously returns EIO back to client
with setattr procedure call (truncate error).

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