bpf_exp: free duplicated labels at exit time

Valgrind found that extracted labels that are passed from the lexer
weren't freed upon exit. Therefore, add a small helper function that
walks label tables and frees them. Since also NULL can be passed to
free(3), we do not need to take care of that here. While at it, fix
up a spacing error in bpf_set_curr_label().

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>