net/appletalk: push down BKL into a atalk_dgram_ops

Making the BKL usage explicit in appletalk makes it more
obvious where it is used, reduces code size and helps
getting rid of the BKL in common code.

I did not analyse how to kill lock_kernel from appletalk
entirely, this will involve either proving that it's not
needed, or replacing with a proper mutex or spinlock,
after finding out which data structures are protected
by the lock.

Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>