datatype: add DTYPE_F_CLONE flag

datatype: add DTYPE_F_CLONE flag

This flag allows us to identify datatypes that are instances from
original datatypes.

This fixes a possible double free when attaching a concatenation
datatype to set->keytype while being also referenced from concatenation
expressions.

ip6/flowtable.t: ERROR: line 5: src/nft add rule --debug=netlink ip6 test-ip6 input flow table acct_out { meta iif . ip6 saddr timeout 600s counter }: This rule should not have failed.
*** Error in `src/nft': double free or corruption (fasttop): 0x000000000117ce70 ***

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>