parser_bison: add selector_expr rule to restrict typeof_expr

parser_bison: add selector_expr rule to restrict typeof_expr

commit 8bbdcb7346788a067968e3aa62ac7e5a670b08af upstream.

typeof_expr allows for symbol, constant and bitwise expressions,
restrict it to selector expressions.

After this patch, input generated by fuzzer is rejected upfront:

 # nft -f test.nft
 test.nft:3:53-53: Error: syntax error, unexpected number
               typeof numgen inc mod 2 : ip daddr . 0
                                                    ^
 test.nft:2:12-13: Error: set definition does not specify key
       map t2 {
           ^^
 test.nft:8:65-67: Error: No such file or directory
               meta l4proto tcp dnat ip to numgen inc mod 2 map @t2
                                                                ^^^
 test.nft:8:65-67: Error: No such file or directory
               meta l4proto tcp dnat ip to numgen inc mod 2 map @t2
                                                                ^^^

Revisit 4ab1e5e60779 ("src: allow use of 'verdict' in typeof
definitions") to handle verdict as string, later a token can be added
to the scanner and enable it via flex start conditions.

Fixes: 14357cff40ed ("parser: add typeof keyword for declarations")
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>