netfilter: nfnetlink_acct: validate NFACCT_FILTER parameters

nfacct_filter_alloc doesn't validate the NFACCT_FILTER_MASK and
NFACCT_FILTER_VALUE parameters which can trigger a NULL pointer
dereference. CAP_NET_ADMIN is required to trigger the bug.

Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>