CHERRY-PICK: UPSTREAM: HID: Separate struct hid_device's driver_lock into two locks.

CHERRY-PICK: UPSTREAM: HID: Separate struct hid_device's driver_lock into two locks.

This patch separates struct hid_device's driver_lock into two. The
goal is to allow hid device drivers to receive input during their
probe() or remove() function calls. This is necessary because some
drivers need to communicate with the device to determine parameters
needed during probe (e.g., size of a multi-touch surface), and if
possible, may perfer to communicate with a device on host-initiated
disconnect (e.g., to put it into a low-power state).

Historically, three functions used driver_lock:

- hid_device_probe: blocks to acquire lock
- hid_device_remove: blocks to acquire lock
- hid_input_report: if locked returns -EBUSY, else acquires lock

This patch adds another lock (driver_input_lock) which is used to
block input from occurring. The lock behavior is now:

- hid_device_probe: blocks to acq. driver_lock, then driver_input_lock
- hid_device_remove: blocks to acq. driver_lock, then driver_input_lock
- hid_input_report: if driver_input_lock locked returns -EBUSY, else
  acquires driver_input_lock

This patch also adds two helper functions to be called during probe()
or remove(): hid_device_io_start() and hid_device_io_stop(). These
functions lock and unlock, respectively, driver_input_lock; they also
make a note of whether they did so that hid-core knows if a driver has
changed the lock state.

This patch results in no behavior change for existing devices and
drivers. However, during a probe() or remove() function call in a
driver, that driver may now selectively call hid_device_io_start() to
let input events come through, then optionally call
hid_device_io_stop() to stop them.

BUG=chromium-os:39354
TEST=manually tested on Link

(Hasn't quite landed upstream, yet, but this has passed review on linux-input)
Signed-off-by: Andrew de los Reyes <adlr@chromium.org>
Change-Id: I737f6fc15911134b51273acf8d3de92fa5cc0f85
Reviewed-on: https://gerrit.chromium.org/gerrit/46004
Reviewed-by: Yufeng Shen <miletus@chromium.org>
Commit-Queue: Andrew de los Reyes <adlr@chromium.org>
Tested-by: Andrew de los Reyes <adlr@chromium.org>