vfs: Initialize "lastfail" in vfs_mountroot_wait()

This variable is only used to rate-limit "Root mount waiting for: ..."
messages using ppsratecheck().

Reported by:	KMSAN
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

sys/kern/vfs_mountroot.c

@@ -983,6 +983,7 @@ vfs_mountroot_wait(void)
 	TSENTER();
 
 	curfail = 0;
+	lastfail.tv_sec = 0;
 	while (1) {
 		g_waitidle();
 		mtx_lock(&root_holds_mtx);