SUNRPC: Trap RDMA segment overflows

Prevent svc_rdma_build_writes() from walking off the end of a Write
chunk's segment array. Caught with KASAN.

The test that this fix replaces is invalid, and might have been left
over from an earlier prototype of the PCL work.

Fixes: 7a1cbfa180 ("svcrdma: Use parsed chunk lists to construct RDMA Writes")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/xprtrdma/svc_rdma_rw.c

@@ -478,10 +478,10 @@ svc_rdma_build_writes(struct svc_rdma_write_info *info,
 		unsigned int write_len;
 		u64 offset;
 
-		seg = &info->wi_chunk->ch_segments[info->wi_seg_no];
-		if (!seg)
+		if (info->wi_seg_no >= info->wi_chunk->ch_segcount)
 			goto out_overflow;
 
+		seg = &info->wi_chunk->ch_segments[info->wi_seg_no];
 		write_len = min(remaining, seg->rs_length - info->wi_seg_off);
 		if (!write_len)
 			goto out_overflow;