nft: Use xtables_{m,c}alloc() everywhere

nft: Use xtables_{m,c}alloc() everywhere

Make use of libxtables allocators where sensible to have implicit error
checking. Leave library-internal calls in place to not create unexpected
program exit points for users, apart from xt_xlate_alloc() as that
function called xtables_error() in error case which exits by itself
already.

Signed-off-by: Phil Sutter <phil@nwl.cc>