gfs2: make function gfs2_make_fs_ro() to void type

It fixes the following warning detected by coccinelle:
./fs/gfs2/super.c:592:5-10: Unneeded variable: "error". Return "0" on
line 628

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>