CAPI: Rework locking of capidev members

Rename 'ncci_list_mtx' to 'lock', expressing that it now protects a
larger set of capidev members: the NCCI list, ap.applid (ie. the
registration of the application), and modifications of userflags.

We do not need to protect each and every check for ap.applid because,
once an application is registered, it will stay for the whole lifetime
of the device.

Also, there is no need to apply the capidev mutex during release (if
there could be concurrent users, we would crash them anyway by freeing
the device at the end of capi_release).

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