NFS/pNFS: Don't leak DS commits in pnfs_generic_retry_commit()

We must ensure that we pass a layout segment to nfs_retry_commit() when
we're cleaning up after pnfs_bucket_alloc_ds_commits(). Otherwise,
requests that should be committed to the DS will get committed to the
MDS.
Do so by ensuring that pnfs_bucket_get_committing() always tries to
return a layout segment when it returns a non-empty page list.

Fixes: c84bea59 ("NFS/pNFS: Simplify bucket layout segment reference counting")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/pnfs_nfs.c

@@ -403,12 +403,16 @@ pnfs_bucket_get_committing(struct list_head *head,
 			   struct pnfs_commit_bucket *bucket,
 			   struct nfs_commit_info *cinfo)
 {
+	struct pnfs_layout_segment *lseg;
 	struct list_head *pos;
 
 	list_for_each(pos, &bucket->committing)
 		cinfo->ds->ncommitting--;
 	list_splice_init(&bucket->committing, head);
-	return pnfs_free_bucket_lseg(bucket);
+	lseg = pnfs_free_bucket_lseg(bucket);
+	if (!lseg)
+		lseg = pnfs_get_lseg(bucket->lseg);
+	return lseg;
 }
 
 static struct nfs_commit_data *
@@ -420,8 +424,6 @@ pnfs_bucket_fetch_commitdata(struct pnfs_commit_bucket *bucket,
 	if (!data)
 		return NULL;
 	data->lseg = pnfs_bucket_get_committing(&data->pages, bucket, cinfo);
-	if (!data->lseg)
-		data->lseg = pnfs_get_lseg(bucket->lseg);
 	return data;
 }