print: skip keys which were removed during iteration
print: skip keys which were removed during iteration

it's possible that the eBPF program removes a key while bpftrace is
iterating over all keys and printing them. This commit skips keys
which were removed during bpf_get_next_key and bpf_lookup_elem.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace