net/packet: remove unused parameter in prb_curr_blk_in_use().

This patch removes unused parameter from prb_curr_blk_in_use() method
in net/packet/af_packet.c.

Signed-off-by: Rami Rosen <rami.rosen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>