netfilter: nft_compat: fix hook validation for non-base chains

Set hook_mask to zero for non-base chains, otherwise people may hit
bogus errors from the xt_check_target() and xt_check_match() when
validating the uninitialized hook_mask.

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

diff --cc net/netfilter/nft_compat.c
Simple merge