rwsem: shorter spinlocked section in rwsem_down_failed_common()

This change reduces the size of the spinlocked and TASK_UNINTERRUPTIBLE
sections in rwsem_down_failed_common():

- We only need the sem->wait_lock to insert ourselves on the wait_list;
  the waiter node can be prepared outside of the wait_lock.

- The task state only needs to be set to TASK_UNINTERRUPTIBLE immediately
  before checking if we actually need to sleep; it doesn't need to protect
  the entire function.

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: move rwsem_down_failed_common code into rwsem_down_{read,write}_failed

Remove the rwsem_down_failed_common function and replace it with two
identical copies of its code in rwsem_down_{read,write}_failed.
This is because we want to make different optimizations in
rwsem_down_{read,write}_failed; we are adding this pure-duplication
step as a separate commit in order to make it easier to check the
following steps.

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: simplify rwsem_down_read_failed

When trying to acquire a read lock, the RWSEM_ACTIVE_READ_BIAS adjustment
doesn't cause other readers to block, so we never have to worry about waking
them back after canceling this adjustment in rwsem_down_read_failed().

We also never want to steal the lock in rwsem_down_read_failed(), so we
don't have to grab the wait_lock either.

Signed-off-by: Michel Lespinasse <walken@google.com>
rwsem: simplify rwsem_down_write_failed

When waking writers, we never grant them the lock - instead, they have
to acquire it themselves when they run, and remove themselves from the
wait_list when they succeed.

As a result, we can do a few simplifications in rwsem_down_write_failed():

- We don't need to check for !waiter.task since __rwsem_do_wake() doesn't
  remove writers from the wait_list

- There is no point releaseing the wait_lock before entering the wait loop,
  as we will need to reacquire it immediately. We can change the loop so
  that the lock is always held at the start of each loop iteration.

- We don't need to get a reference on the task structure, since the task
  is responsible for removing itself from the wait_list. There is no risk,
  like in the rwsem_down_read_failed() case, that a task would wake up and
  exit (thus destroying its task structure) while __rwsem_do_wake() is
  still running - wait_lock protects against that.

Signed-off-by: Michel Lespinasse <walken@google.com>