VOP_LOOKUP.9: update locking info

[asomers]

Summary

VOP_LOOKUP.9: update locking info

Test Plan

This change was originally submitted to our private repo by Will, with a
comment that it originally came from kib. I don't know anything else about it.
If you two still think it's correct, I'll commit it.

[kib]

Yes, the new statement is correct but perhaps is somewhat vague.

You may look at the kern/vnode_if.src to see that we do assert that dvp is locked on VOP_LOOKUP() entry and is kept locked on both successful and erroneous returns. But depending on the arguments, we might upgrade/downgrade the lock as needed.