netfilter: fix sparse warnings in reject handling

netfilter: fix sparse warnings in reject handling

make C=1 CF=-D__CHECK_ENDIAN__ shows following:

net/bridge/netfilter/nft_reject_bridge.c:65:50: warning: incorrect type in argument 3 (different base types)
net/bridge/netfilter/nft_reject_bridge.c:65:50:    expected restricted __be16 [usertype] protocol [..]
net/bridge/netfilter/nft_reject_bridge.c:102:37: warning: cast from restricted __be16
net/bridge/netfilter/nft_reject_bridge.c:102:37: warning: incorrect type in argument 1 (different base types) [..]
net/bridge/netfilter/nft_reject_bridge.c:121:50: warning: incorrect type in argument 3 (different base types) [..]
net/bridge/netfilter/nft_reject_bridge.c:168:52: warning: incorrect type in argument 3 (different base types) [..]
net/bridge/netfilter/nft_reject_bridge.c:233:52: warning: incorrect type in argument 3 (different base types) [..]

Caused by two (harmless) errors:
1. htons() instead of ntohs()
2. __be16 for protocol in nf_reject_ipXhdr_put API, use u8 instead.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>