Pid namespace support

[pagict]

inspired by the

bcc/tools/profile.py

Line 168 in 6acb86e

struct bpf_pidns_info ns = {};

if the cpudist runs in a pid namespace, the filtered pid/tgid are not as expected.
since the kernel does not provide something like iint bpf_get_ns_pid_tgid(u64 dev, u64 ino, struct bpf_pidns_info * ns_data, u32 size), we can create the mapping for the current, then find out the mapped ns pid by the real pid whenever the prev is refered.