x86 rwsem: avoid taking slow path when stealing write lock

modify __down_write[_nested] and __down_write_trylock to grab the write
lock whenever the active count is 0, even if there are queued waiters
(they must be writers pending wakeup, since the active count is 0).

Note that this is an optimization only; architectures without this
optimization will still work fine:

- __down_write() would take the slow path which would take the wait_lock
  and then try stealing the lock (as in the spinlocked rwsem implementation)

- __down_write_trylock() would fail, but callers must be ready to deal
  with that - since there are some writers pending wakeup, they could
  have raced with us and obtained the lock before we steal it.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>