Fix readdir for .zfs/snapshot directory

dmu_snapshot_list_next stores the index of the next snapshot entry to the offp
argument, which zpl_snapdir_iterate then uses for the dir_emit. This
result in an off-by-one error. Therefore a temporary variable should be
used.

This was a regression introduced in commit openzfs/zfs@0f37d0c.

Signed-off-by: Andrey Vesnovaty <andrey.vesnovaty@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#2930

module/zfs/zpl_ctldir.c

@@ -252,25 +252,28 @@ zpl_snapdir_iterate(struct file *filp, struct dir_context *ctx)
 	zfs_sb_t *zsb = ITOZSB(filp->f_path.dentry->d_inode);
 	char snapname[MAXNAMELEN];
 	boolean_t case_conflict;
-	uint64_t id;
+	uint64_t id, cookie;
 	int error = 0;
 
 	ZFS_ENTER(zsb);
 
 	if (!dir_emit_dots(filp, ctx))
 		goto out;
 
+	cookie = ctx->pos;
 	while (error == 0) {
 		dsl_pool_config_enter(dmu_objset_pool(zsb->z_os), FTAG);
 		error = -dmu_snapshot_list_next(zsb->z_os, MAXNAMELEN,
-		    snapname, &id, &ctx->pos, &case_conflict);
+		    snapname, &id, &cookie, &case_conflict);
 		dsl_pool_config_exit(dmu_objset_pool(zsb->z_os), FTAG);
 		if (error)
 			goto out;
 
 		if (!dir_emit(ctx, snapname, strlen(snapname),
 		    ZFSCTL_INO_SHARES - id, DT_DIR))
 			goto out;
+
+		ctx->pos = cookie;
 	}
 out:
 	ZFS_EXIT(zsb);