netfilter: nft_payload: sanitize offset and length before calling skb_checksum()

[ Upstream commit d5953d680f7e96208c29ce4139a0e38de87a57fe ]

If access to offset + length is larger than the skbuff length, then
skb_checksum() triggers BUG_ON().

skb_checksum() internally subtracts the length parameter while iterating
over skbuff, BUG_ON(len) at the end of it checks that the expected
length to be included in the checksum calculation is fully consumed.

Fixes: 7ec3f7b4 ("netfilter: nft_payload: add packet mangling support")
Reported-by: Slavin Liu <slavin-ayu@qq.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit a661ed364ae6ae88c2fafa9ddc27df1af2a73701)
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>