xtables-save: Fix table compatibility check

xtables-save: Fix table compatibility check

The builtin table check guarding the 'is incompatible' warning was
wrong: The idea was to print the warning only for incompatible tables
which are builtin, not for others. Yet the code would print the warning
only for non-builtin ones.

Also reorder the checks: nft_table_builtin_find() is fast and therefore
a quick way to bail for uninteresting tables. The compatibility check is
needed for the remaining tables, only.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>