irqdomain: Fix mapping-creation race

Parallel probing (e.g. due to asynchronous probing) of devices that share
interrupts can currently result in two mappings for the same hardware
interrupt to be created.

Add a serialising mapping mutex so that looking for an existing mapping
before creating a new one is done atomically.

Note that serialising the lookup and creation in
irq_create_mapping_affinity() would have been enough to prevent the
duplicate mapping, but that could instead cause
irq_create_fwspec_mapping() to fail when there is a race.

Fixes: 765230b5 ("driver-core: add asynchronous probing support for drivers")
Fixes: b62b2cf5 ("irqdomain: Fix handling of type settings for existing mappings")
Cc: Dmitry Torokhov <dtor@chromium.org>
Cc: Jon Hunter <jonathanh@nvidia.com>
Link: https://lore.kernel.org/r/YuJXMHoT4ijUxnRb@hovoldconsulting.com


Signed-off-by: Johan Hovold <johan+linaro@kernel.org>