6lowpan: double unlock on an error path

We already unlocked a few lines earlier here, so we can go directly to
drop without passing through unlock.  This was introduced recently in
c5d3687f6c ('6lowpan: read data from skb safely').

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>