sched/wait: Replace wait_on_atomic_t() with wait_var_event()

I never really liked the whole wait_on_atomic_t() because of the hard-coded type and condition. And when Dan proposed to add 50+ lines of duplication to implement wait_on_atomic_one() I figured that was it.

Instead implement wait_var_event(), which at the fundamental level is much the same hashed-waitqueue as wait_on_atomic_t() is, but it doesn't care about the type or condition.

It is further modelled on the existing wait_event() API and is otherwise consistent with that.

The existing:

  wait_for_atomic_t(&my_atomic, wait_atomic_t, TASK_UNINTERRUPTIBLE);
  wake_up_atomic_t(&my_atomic);

can be replaced with:

  wait_var_event(&my_atomic, !atomic_read(&my_atomic));
  wake_up_var(&my_atomic);

And Dan can do what he needs done using:

  wait_var_event(&my_atomic, atomic_read(&my_atomic) == 1);

---
 arch/mips/kernel/process.c                         |   2 +
 arch/mips/kernel/traps.c                           |   4 +-
 drivers/gpu/drm/drm_dp_aux_dev.c                   |  13 +--
 drivers/gpu/drm/i915/selftests/intel_breadcrumbs.c |  14 +--
 drivers/media/platform/qcom/venus/hfi.c            |   8 +-
 fs/afs/cell.c                                      |   6 +-
 fs/afs/rxrpc.c                                     |   6 +-
 fs/afs/server.c                                    |   6 +-
 fs/btrfs/extent-tree.c                             |  14 ++-
 fs/btrfs/ioctl.c                                   |   2 +-
 fs/fscache/cookie.c                                |   7 +-
 fs/nfs/inode.c                                     |   5 -
 fs/nfs/pagelist.c                                  |   6 +-
 fs/nfs/pnfs_nfs.c                                  |   2 +-
 fs/nfs/write.c                                     |   6 +-
 fs/ocfs2/filecheck.c                               |   9 +-
 include/linux/fscache-cache.h                      |   2 +-
 include/linux/wait_bit.h                           |  95 +++++++++++++-----
 kernel/sched/wait_bit.c                            | 110 +++++----------------
 19 files changed, 147 insertions(+), 170 deletions(-)