vfs: fix early termination of kern_getfsstat

The kernel would unlock already unlocked mutex if the buffer got filled up
before the mount list ended.

Reported by:	pho
Fixes:	r363069 ("vfs: depessimize getfsstat when only the count is requested")

sys/kern/vfs_syscalls.c

@@ -551,7 +551,7 @@ kern_getfsstat(struct thread *td, struct statfs **buf, size_t bufsize,
 
 		if (count == maxcount) {
 			vfs_unbusy(mp);
-			break;
+			goto out;
 		}
 
 		mtx_lock(&mountlist_mtx);