netfilter: nft_payload: use __be16 to store gre version

GRE_VERSION and GRE_VERSION0 are expressed in network byte order,
use __be16. Uncovered by sparse:

net/netfilter/nft_payload.c:112:25: warning: incorrect type in assignment (different base types)
net/netfilter/nft_payload.c:112:25:    expected unsigned int [usertype] version
net/netfilter/nft_payload.c:112:25:    got restricted __be16
net/netfilter/nft_payload.c:114:22: warning: restricted __be16 degrades to integer

Fixes: c247897d

 ("netfilter: nft_payload: access GRE payload via inner offset")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>