bpf: move global verifier log into verifier environment

The biggest piece of global state protected by the verifier lock
is the verifier_log.  Move that log to struct bpf_verifier_env.
struct bpf_verifier_env has to be passed now to all invocations
of verbose().

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>