5.x locking plan

My curiousity has overcome my fear of the bikeshed so I'll ask the
question that has been bugging me for a while.  Why haven't we gone
through the tree and created a lock for each spl and then converted every
spl call into the appropriate mtx_lock call?  At that point, we can mark
large sections of the tree giant-free and then make the locking data-based
(instead of code-based) one section at a time.  This is the approach
Solaris took.

-Nate