sx locks rewriting - needs testers

[Attilio Rao]

In my P4 branch: //depot/user/attilio/attilio_smpng/...
you can find a sx locks rewriting using the optimized semantic of
rwlocks; in the end this might result in a valuable performance
improvement.

Some hints about it:
- new sx locks alredy support inlined s*lock operations and try* operations;
  (they have a fully functional support)
- new sx locks doesn't have support for adaptive spinning yet; this is beacause
   the code is under revision even for mutex/rwlock.
- we could allow a sharers tracking in debugging mode, at least, in
order to detect
  eventual recursion in slock operation
- currently, sx locks mantain the exclusive holder tracking even if this is not
   really necessary (we can get rid of it).

In this moment a strong phase of test is *very* welcome, so please,
for every people having a p4 account, dowload the kernel and try to
put it under stress (at this purpose I cc'ed, in particular, kris@,
pho@ and jhb@ in order to have tests, revisions, etc. etc.)

Note1: the branch contains other diffs respect CURRENT.
Note2: due to same strange struct sx members accesses, I had to
disable XFS locking primitives in order to not hurt the current work.
This is possibly buggy...

Thanks,
Attilio

-- 
Peace can only be achieved by understanding - A. Einstein