RDS: Add a warning if trying to allocate 0 sgs

rds_message_alloc_sgs() only works when nents is nonzero.

Signed-off-by: Andy Grover <andy.grover@oracle.com>
diff --git a/net/rds/message.c b/net/rds/message.c
index f4c3be0..1f73a73 100644
--- a/net/rds/message.c
+++ b/net/rds/message.c
@@ -240,6 +240,7 @@
 	struct scatterlist *sg_ret;
 
 	WARN_ON(rm->m_used_sgs + nents > rm->m_total_sgs);
+	WARN_ON(!nents);
 
 	sg_ret = &sg_first[rm->m_used_sgs];
 	sg_init_table(sg_ret, nents);