CAPI: Silence lockdep warning on get_capi_appl_by_nr usage

As long as we hold capi_controller_lock, we can safely access
capi_applications without RCU protection as no one can modify the
application list underneath us. Introduce an RCU-free
__get_capi_appl_by_nr for this purpose. This silences lockdep warnings
on suspicious rcu_dereference usage.

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