gfs2: check sb_min_blocksize return value

Check the return value of sb_min_blocksize(): it will be 0 when the
requested block size is invalid.

In addition, check the return value of sb_set_blocksize() as well.

Reported-by:  <syzbot+b0018b7468b2af33b4d5@syzkaller.appspotmail.com>
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>