Use correct NET_RX_* returns for atalk_rcv()

In all rx'd SKB cases, atalk_rcv() either eventually jumps to or falls through
    to the label out:, which  returns numeric 0. Numeric 0 corresponds to
    NET_RX_SUCCESS, which is incorrect in failed SKB cases.

    This patch makes atalk_rcv() provide the correct returns by:

    o  explicitly returning NET_RX_SUCCESS in the two success cases
    o  having the out: label return NET_RX_DROP, instead of numeric 0
    o  making the failed SKB labels and processing more consistent with other
       _rcv() routines in the kernel, simplifying validation and removing a
       backwards goto

Signed-off-by: Mark Smith <markzzzsmith@yahoo.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>