netfilter: nft_compat: run xt_check_hooks_{match,target}() from .validate

netfilter: nft_compat: run xt_check_hooks_{match,target}() from .validate

Several matches and one target check that the hook is correct from
checkentry(), however, the basechain is only available from
nft_table_validate().

This patch uses xt_check_hooks_{match,target}() from the nft_compat
expression .validate path.

This patch sets the table in the nft_ctx struct in nft_table_validate()
which is required by this patch.

Based on patch from Florian Westphal.

Fixes: 0ca743a55991 ("netfilter: nf_tables: add compatibility layer for x_tables")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>