gfs2: Fix kernel NULL pointer dereference in gfs2_rgrp_dump

gfs2: Fix kernel NULL pointer dereference in gfs2_rgrp_dump

Syzkaller has reported a NULL pointer dereference when accessing
rgd->rd_rgl in gfs2_rgrp_dump().  This can happen when creating
rgd->rd_gl fails in read_rindex_entry().  Add a NULL pointer check in
gfs2_rgrp_dump() to prevent that.

Reported-and-tested-by: syzbot+da0fc229cc1ff4bb2e6d@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=da0fc229cc1ff4bb2e6d
Fixes: 72244b6bc752 ("gfs2: improve debug information when lvb mismatches are found")
Signed-off-by: Osama Muhammad <osmtendev@gmail.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>