CAPI: Remove experimental tag from middleware feature

Despite all its bugs, the middleware support of our CAPI stack was
already in use for many, many moons. And after going through its code,
fixing all issues I found, I feel it deserves to officially become a
non-experimental feature.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>