netfilter: nf_tables: fix nft_trans type confusion

nft_trans_FOO objects all share a common nft_trans base structure, but
trailing fields depend on the real object size. Access is only safe after
trans->msg_type check.

Check for rule type first.  Found by code inspection.

Fixes: 1a94e38d

 ("netfilter: nf_tables: add NFTA_RULE_ID attribute")
Signed-off-by: Florian Westphal <fw@strlen.de>