netfilter: nft_meta: split nft_meta_init() into two functions for get/set

For value spanning multiple registers, we need to validate the length
of data loads. In order to add this to nft_meta, we need the length from
key validation. Split the nft_meta_init() function into two functions
for the get and set operations as preparation for that.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@gnumonks.org>