gpiolib: hold gpio devices lock until ->descs array is initialised

gpiolib: hold gpio devices lock until ->descs array is initialised

If a driver consuming the GPIO chip is being probed at the same time as
the GPIO driver is registering the chip, it is possible for the
consuming driver to see the ->descs array in an uninitialised state.
For example, the gpio-keys-polled driver can fail like this:

    kernel: gpiod_request: invalid GPIO (no device)
    kernel: gpio-keys-polled PRP0001:07: failed to get gpio: -22
    kernel: gpio-keys-polled: probe of PRP0001:07 failed with error -22

This patch makes gpiochip_add() hold the lock protecting gpio_devices
until it has finished setting desc->gdev on the newly inserted list
entry.

Signed-off-by: Dan Callaghan <dan.callaghan@opengear.com>
Link: https://lore.kernel.org/r/20200121001216.15964-1-dan.callaghan@opengear.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>