[PACKET]: Fix sparse warnings in af_packet.c

  CHECK   net/packet/af_packet.c
net/packet/af_packet.c:1876:14: warning: context imbalance in 'packet_seq_start' - wrong count at exit
net/packet/af_packet.c:1888:13: warning: context imbalance in 'packet_seq_stop' - unexpected unlock

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>