netfilter: nfnetlink_acct: fix nfnl_acct_get operation

The get operation was not sending the message that was built to
user-space. This patch also includes the appropriate handling for
the return value of netlink_unicast().

Moreover, fix error codes on error (for example, for non-existing
entry was uncorrect).

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>