bpf: Reset data_meta before running programs attached to devmap entry

This is a new context that does not handle metadata at the moment, so
mark data_meta invalid.

Fixes: fbee97fe ("bpf: Add support to attach bpf program to a devmap entry")
Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200608151723.9539-1-dsahern@kernel.org