When mounting a file system re-lookup and lock the directory we mount on
after the file system is setup by VFS_MOUNT().  This way recursive vnode
locks are no longer needed here and mounts on null mounts no longer fail
as described in PR #43439 (mount_null panic: lockdebug_wantlock: locking
against myself).

Based on a proposal from  and
Reviewed by: David A. Holland <dholland@netbsd.org>

(hannken)