Current semantics, if I understand correctly:

/*
 * lookup_once(state, searchdir, newsearchdir_ret, foundobj_ret)
 *
 *	Look up one component in searchdir: try VOP_LOOKUP on each
 *	mount in the stack of union mounts until one answers
 *	positively.  If a file system is mounted on the found object,
 *	instead find the root of that file system.
 *
 *	On entry, searchdir must be locked and referenced.  As
 *	lookup_once iteratively attempts different search directories,
 *	it will unlock and release the previous one, starting at
 *	searchdir, and lock and reference the next one.
 *
 *	Sets *newsearchdir_ret to be the locked and referenced last
 *	directory searched, whether successfully or not; or to be the
 *	found object if it is the root of a file system.  Sets
 *	*foundobj_ret to be the locked and referenced found object, or
 *	NULL if none was found.
 */

Proposed patch semantics, if I understand correctly:

/*
 * lookup_once(state, searchdir, newsearchdir_ret, foundobj_ret)
 *
 *	Look up one component in searchdir: try VOP_LOOKUP on each
 *	mount in the stack of union mounts until one answers
 *	positively.  If a file system is mounted on the found object,
 *	instead find the root of that file system.
 *
 *	On entry, searchdir must be locked and referenced.  As
 *	lookup_once iteratively attempts different search directories,
 *	it will unlock and release the previous one, starting at
 *	searchdir, and lock and reference the next one.
 *
 *	Sets *newsearchdir_ret to be the locked and referenced last
 *	directory searched, whether successfully or not; or NULL, if
 *	the found object is a mount point.  Sets *foundobj_ret to be
 *	the locked and referenced found object, or NULL if none was
 *	found.
 */

Proposed clearer semantics:

/*
 * lookup_once(state, searchdir, newsearchdir_ret, foundobj_ret)
 *
 *	Look up one component in searchdir: try VOP_LOOKUP on each
 *	mount in the stack of union mounts until one answers
 *	positively.  If a file system is mounted on the found object,
 *	instead find the root of the file system.
 *
 *	On entry, searchdir must be locked and referenced.  As
 *	lookup_once iteratively attempts different search directories,
 *	it will unlock and release the previous one, starting at
 *	searchdir, and lock and reference the next one.
 *
 *	On success, sets *newsearchdir_ret to be the locked and
 *	referenced actual directory where the object was found, or NULL
 *	if the found object is the root of a mounted file system; and
 *	sets *foundobj_ret to be the locked and referenced found object.
 *
 *	On failure, unlocks and releases searchdir, and sets
 *	*newsearchdir_ret and *foundobj_ret to NULL.
 */