gfs2: Fix NULL pointer dereference in gfs2_rgrp_dump

When an rindex entry is found to be corrupt, compute_bitstructs() calls
gfs2_consist_rgrpd() which calls gfs2_rgrp_dump() like this:

    gfs2_rgrp_dump(NULL, rgd->rd_gl, fs_id_buf);

gfs2_rgrp_dump then dereferences the gl without checking it and we get

    BUG: KASAN: null-ptr-deref in gfs2_rgrp_dump+0x28/0x280

because there's no rgrp glock involved while reading the rindex on mount.

Fix this by changing gfs2_rgrp_dump to take an rgrp argument.

Reported-by:  <syzbot+43fa87986bdd31df9de6@syzkaller.appspotmail.com>
Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>