gfs2: Fix kernel NULL pointer dereference in gfs2_rgrp_dump

In the Linux kernel, the following vulnerability has been resolved: 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.