Streamline the operations when acquiring the lock on an INP after a lookup

[jtl]

Summary

A few of the INP lookup operations that lock INPs after the lookup do so using this mechanism (to maintain lock ordering):

1. Lock lookup structure.
2. Find INP.
3. Acquire reference on INP.
4. Drop lock on lookup structure.
5. Acquire INP lock.
6. Drop reference on INP.

This change provides a slightly shorter path for cases where the INP lock is uncontested:

1. Lock lookup structure.
2. Find INP.
3. Try to acquire the INP lock.
4. If successful, drop lock on lookup structure.

Of course, if the INP lock is contested, the functions will need to revert to the previous way of switching locks safely.

Test Plan

Tested at high scale with no ill effects.

[rwatson]

I like this change, and have not yet spotted any problems with it. The one change I might make is to add a few assertions before the final return(inp) in each case to assert that the held lock matches the requested lock.

[jtl]

Add assertions suggested by @rwatson.