Phonet: mark the pipe controller as EXPERIMENTAL

There are a bunch of issues that need to be fixed, including:
 - GFP_KERNEL allocations from atomic context
   (and GFP_ATOMIC in process context),
 - abuse of the setsockopt() call convention,
 - unprotected/unlocked static variables...

IMHO, we will need to alter the userspace ABI when we fix it. So mark
the configuration option as EXPERIMENTAL for the time being (or should
it be BROKEN instead?).

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>