bcache: remove EXPERIMENTAL for Kconfig option 'Asynchronous device registration'

The "Asynchronous device registration (EXPERIMENTAL)" Kconfig option is
for 2+ years, it is used when registration takes too much time for
massive amount of cached data, to avoid udev task timeout during boot
time.

Many users and products enable this Kconfig option for quite long time
(e.g. SUSE Linux) and it works as expected and no issue reported.

It is time to remove the "EXPERIMENTAL" tag from this Kconfig item.

Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20220719042724.8498-2-colyli@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>