zfs receive: -u can be ignored sometimes

When force-receiving a filesystem that was already mounted the re-created
filesystem is mounted despite -u flag.

Also see https://www.illumos.org/issues/6412.

PR:		204705
Tested by:	Vladimir Krstulja <vlad-fbsd@acheronmedia.com>
MFC after:	2 weeks
X-Needs-Upstreaming:	illumos

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c

@@ -3479,7 +3479,8 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 	}
 
 	if (clp) {
-		err |= changelist_postfix(clp);
+		if (!flags->nomount)
+			err |= changelist_postfix(clp);
 		changelist_free(clp);
 	}