gfs2: Turn gfs2_withdraw into a void function

Since commit 601ef0d5 ("gfs2: Force withdraw to replay journals and
wait for it to finish"), gfs2_withdraw() always returns -1, so turn it
into a straight void function.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>