nfs: fix DIO good bytes calculation

For direct read that has IO size larger than rsize, we'll split
it into several READ requests and nfs_direct_good_bytes() would
count completed bytes incorrectly by eating last zero count reply.

Fix it by handling mirror and non-mirror cases differently such that
we only count mirrored writes differently.

This fixes 5fadeb47("nfs: count DIO good bytes correctly with mirroring").
Reported-by: Jean Spector <jean@primarydata.com>
Cc: <stable@vger.kernel.org> # v3.19+
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/direct.c

@@ -131,12 +131,16 @@ nfs_direct_good_bytes(struct nfs_direct_req *dreq, struct nfs_pgio_header *hdr)
 
 	WARN_ON_ONCE(hdr->pgio_mirror_idx >= dreq->mirror_count);
 
+	if (dreq->mirror_count == 1) {
+		dreq->mirrors[hdr->pgio_mirror_idx].count += hdr->good_bytes;
+		dreq->count += hdr->good_bytes;
+	} else {
+		/* mirrored writes */
 		count = dreq->mirrors[hdr->pgio_mirror_idx].count;
 		if (count + dreq->io_start < hdr->io_start + hdr->good_bytes) {
 			count = hdr->io_start + hdr->good_bytes - dreq->io_start;
 			dreq->mirrors[hdr->pgio_mirror_idx].count = count;
 		}
-
 		/* update the dreq->count by finding the minimum agreed count from all
 		 * mirrors */
 		count = dreq->mirrors[0].count;
@@ -145,6 +149,7 @@ nfs_direct_good_bytes(struct nfs_direct_req *dreq, struct nfs_pgio_header *hdr)
 			count = min(count, dreq->mirrors[i].count);
 
 		dreq->count = count;
+	}
 }
 
 /*