expression: fix output of verdict maps

expression: fix output of verdict maps

% nft list table filter
table ip filter {
	...
	chain output {
		...
		ip saddr map { 1.1.1.1 => accept}
	}
}

It displays 'map' instead of 'vmap'. Fix it by checking the mapping
type in map_expr_print().

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