netfilter: nft_byteorder: avoid unneeded le/be conversion steps

David points out that we to three le/be conversions instead
of just one.  Doesn't matter on x86_64 w. gcc, but other
architectures might be less lucky.

Since it also simplifies code just follow his advice.

Fixes: c0f3275f5cb ("nftables: byteorder: provide le/be 64 bit conversion helper")
Suggested-by: David Laight <David.Laight@aculab.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>