Remove the assertion "searchdir != foundobj" from lookup_crossmount().

It will trigger whenever we lookup "." on a directory that becomes
mounted

    fd = open("/mnt")
    mount(..., "/mnt", ...)
    fd2 = openat(fd, ".")

or

    Unlocked searchdir gets mounted mounted on between
    lookup_fastforward()/lookup_once() and the test for
    foundobj->v_mountedhere.

May address syzkaller:

Reported-by: syzbot+9197ac681ce50f707d9a@syzkaller.appspotmail.com
Reported-by: syzbot+eb4854df8ee3c9bc278d@syzkaller.appspotmail.com
Reported-by: syzbot+3cc5b4126ab554f145d3@syzkaller.appspotmail.com
Reported-by: syzbot+7eae48a3ea952efee8c8@syzkaller.appspotmail.com
Reported-by: syzbot+b7f662083ccf8be3e669@syzkaller.appspotmail.com

sys/kern/vfs_lookup.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: vfs_lookup.c,v 1.230 2021/11/13 14:52:08 hannken Exp $	*/
+/*	$NetBSD: vfs_lookup.c,v 1.231 2022/02/10 10:59:12 hannken Exp $	*/
 
 /*
  * Copyright (c) 1982, 1986, 1989, 1993
@@ -37,7 +37,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_lookup.c,v 1.230 2021/11/13 14:52:08 hannken Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_lookup.c,v 1.231 2022/02/10 10:59:12 hannken Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_magiclinks.h"
@@ -940,8 +940,6 @@ lookup_crossmount(struct namei_state *state,
 	while (foundobj->v_type == VDIR &&
 	    (mp = foundobj->v_mountedhere) != NULL &&
 	    (cnp->cn_flags & NOCROSSMOUNT) == 0) {
-		KASSERTMSG(searchdir != foundobj, "same vn %p", searchdir);
-
 		/*
 		 * Try the namecache first.  If that doesn't work, do
 		 * it the hard way.