support run multiple ktap instances concurrently

This patch move percpu global variables and tstring_lock
into ktap_global_state, so we can run multiple ktap
instances concurrently now.
(global percpu variable will make memory corruption,
and global tstring_lock will make system deadlock)

Performance impact(a little overhead):
tesed by test/bench

Before:
247360
247294
248100

After:
243764
242612
243406

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
fix pid() bug when running ktap concurrently

A kernel oops bug found when running two ktap instances concurrently
using below ktap one-liner.

	ktap -e "trace *:* {print(pid()}"

When one ktap instance exited, kernel oops.

 174 [  471.611139] Call Trace:
 175 [  471.611139]  [<ffffffffa004a414>] ktap_lib_pid+0x24/0x50 [ktapvm]
 176 [  471.611139]  [<ffffffffa0045d70>] precall+0xf0/0x320 [ktapvm]
 177 [  471.611139]  [<ffffffffa0046861>] kp_call+0x6d1/0x1950 [ktapvm]
 178 [  471.611139]  [<ffffffffa0046722>] ? kp_call+0x592/0x1950 [ktapvm]
 179 [  471.611139]  [<ffffffffa004a4a0>] ? ktap_lib_print_trace_clock+0x60/0x60      [ktapvm]
 180 [  471.611139]  [<ffffffffa004b0ee>] ktap_overflow_callback+0x10e/0x150 [kt     apvm]
 181 [  471.611139]  [<ffffffff8111fc02>] __perf_event_overflow+0x92/0x270
 182 [  471.611139]  [<ffffffff8111fc02>] ? __perf_event_overflow+0x92/0x270
 183 [  471.611139]  [<ffffffff8111ff71>] perf_swevent_overflow+0x51/0xe0
 184 [  471.611139]  [<ffffffff8112005f>] perf_swevent_event+0x5f/0x90
 185 [  471.611139]  [<ffffffff81120167>] perf_tp_event+0xd7/0x220
 186 [  471.611139]  [<ffffffff8112005f>] ? perf_swevent_event+0x5f/0x90
 187 [  471.611139]  [<ffffffff810b84e4>] perf_trace_lock_acquire+0x134/0x150
 188 [  471.611139]  [<ffffffff810bc848>] ? lock_acquire+0xf8/0x110
 189 [  471.611139]  [<ffffffff810bc848>] lock_acquire+0xf8/0x110
 190 [  471.611139]  [<ffffffff810722c2>] ? free_pid+0x22/0x150
 191 [  471.611139]  [<ffffffff8161c64c>] _raw_spin_lock_irqsave+0x3c/0x50
 192 [  471.611139]  [<ffffffff810722c2>] ? free_pid+0x22/0x150
 193 [  471.611139]  [<ffffffff810722c2>] free_pid+0x22/0x150
 194 [  471.611139]  [<ffffffff8107244a>] __change_pid+0x5a/0x60
 195 [  471.611139]  [<ffffffff81072970>] detach_pid+0x10/0x20
 196 [  471.611139]  [<ffffffff810538c3>] release_task+0x233/0x460
 197 [  471.611139]  [<ffffffff810536a9>] ? release_task+0x19/0x460
 198 [  471.611139]  [<ffffffff810ba23d>] ? trace_hardirqs_on_caller+0xfd/0x1c0
 199 [  471.611139]  [<ffffffff810540de>] do_exit+0x5ee/0xa20
 200 [  471.611139]  [<ffffffff810e3bfc>] ? __audit_syscall_entry+0x9c/0xf0
 201 [  471.611139]  [<ffffffff8100f44d>] ? syscall_trace_enter+0x21d/0x230
 202 [  471.611139]  [<ffffffff810553c7>] SyS_exit+0x17/0x20
 203 [  471.611139]  [<ffffffff81625524>] tracesys+0xdd/0xe2
 204 [  471.611139] Code: 00 55 65 48 8b 04 25 00 ba 00 00 48 8b 80 40 03 00 00      48 89 e5 48 85 c0 74 3a 8b 50 04 48 c1 e2 05 48 85 ff 48 8b 54 10 38 74 31      <8b> 8a 20 08 00 00 31 c0 3b 4f 04 77 0f 48 c1 e1 05 48 8d 4c 0f
 205 [  471.611139] RIP  [<ffffffff81071e0f>] pid_vnr+0x2f/0x70

The reason is ns argument is NULL when tracing callback called in free_pid,
to fix this problem, use current->pid instead of task_tgid_vnr(current).

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>