netfilter: nft_exthdr: fix register tracking for F_PRESENT flag

netfilter: nft_exthdr: fix register tracking for F_PRESENT flag

nft_exthdr_init() passes user-controlled priv->len to
nft_parse_register_store(), which marks that many bytes in the
register bitmap as initialized.  However, when NFT_EXTHDR_F_PRESENT
is set, the eval paths write only 1 byte (nft_reg_store8) or
4 bytes (*dest = 0 on TCP/DCCP error path).  When len > 4,
registers beyond the first are never written, retaining
uninitialized stack data from nft_regs.

Bail out if userspace requests too much data when F_PRESENT is set.

Reported-by: Ji'an Zhou <eilaimemedsnaimel@gmail.com>
Fixes: c078ca3b0c5b ("netfilter: nft_exthdr: Add support for existence check")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>