in pthread_mutex_trylock, EBUSY out more directly when possible

avoid gratuitously setting up and tearing down the robust list pending
slot.

src/thread/pthread_mutex_trylock.c

@@ -15,6 +15,7 @@ int __pthread_mutex_trylock_owner(pthread_mutex_t *m)
 		return 0;
 	}
 	if (own == 0x7fffffff) return ENOTRECOVERABLE;
+	if (own && (!(own & 0x40000000) || !(type & 4))) return EBUSY;
 
 	if (m->_m_type & 128) {
 		if (!self->robust_list.off) {
@@ -25,8 +26,7 @@ int __pthread_mutex_trylock_owner(pthread_mutex_t *m)
 		self->robust_list.pending = &m->_m_next;
 	}
 
-	if ((own && (!(own & 0x40000000) || !(type & 4)))
-	    || a_cas(&m->_m_lock, old, tid) != old) {
+	if (a_cas(&m->_m_lock, old, tid) != old) {
 		self->robust_list.pending = 0;
 		return EBUSY;
 	}