GFS2: return error if malloc failed in gfs2_rs_alloc()

The error code in gfs2_rs_alloc() is set to ENOMEM when error
but never be used, instead, gfs2_rs_alloc() always return 0.
Fix to return 'error'.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/rgrp.c

@@ -576,7 +576,7 @@ int gfs2_rs_alloc(struct gfs2_inode *ip)
 	RB_CLEAR_NODE(&ip->i_res->rs_node);
 out:
 	up_write(&ip->i_rw_mutex);
-	return 0;
+	return error;
 }
 
 static void dump_rs(struct seq_file *seq, const struct gfs2_blkreserv *rs)