decnet: fix sparse warnings: context imbalance

Impact: Attribute functions with __acquires(...) resp. __releases(...).

Fix this sparse warnings:
  net/decnet/dn_dev.c:1324:13: warning: context imbalance in 'dn_dev_seq_start' - wrong count at exit
  net/decnet/dn_dev.c:1366:13: warning: context imbalance in 'dn_dev_seq_stop' - unexpected unlock

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>