nft: cache: Annotate faked base chains as such

nft: cache: Annotate faked base chains as such

To avoid pointless kernel ruleset modifications without too many
workarounds in user space, code sometimes adds "fake" base chains to
cache. Yet these fake entries happen to prevent base chain creation for
a following command which actually requires them. Fix this by annotating
the fake entries as such so *_builtin_init() functions may convert them
into real ones.

Fixes: fd4b9bf08b9eb ("nft: Avoid pointless table/chain creation")
Signed-off-by: Phil Sutter <phil@nwl.cc>