bpf: test_bpf: turn off preemption in function __run_once

bpf: test_bpf: turn off preemption in function __run_once

When running BPF test suite the following splat occurs:

[  415.930950] test_bpf: #0 TAX jited:0
[  415.931067] BUG: assuming atomic context at lib/test_bpf.c:6674
[  415.946169] in_atomic(): 0, irqs_disabled(): 0, pid: 11556, name: modprobe
[  415.953176] INFO: lockdep is turned off.
[  415.957207] CPU: 1 PID: 11556 Comm: modprobe Tainted: G        W         5.0.0-rc7-next-20190220 #1
[  415.966328] Hardware name: HiKey Development Board (DT)
[  415.971592] Call trace:
[  415.974069]  dump_backtrace+0x0/0x160
[  415.977761]  show_stack+0x24/0x30
[  415.981104]  dump_stack+0xc8/0x114
[  415.984534]  __cant_sleep+0xf0/0x108
[  415.988145]  test_bpf_init+0x5e0/0x1000 [test_bpf]
[  415.992971]  do_one_initcall+0x90/0x428
[  415.996837]  do_init_module+0x60/0x1e4
[  416.000614]  load_module+0x1de0/0x1f50
[  416.004391]  __se_sys_finit_module+0xc8/0xe0
[  416.008691]  __arm64_sys_finit_module+0x24/0x30
[  416.013255]  el0_svc_common+0x78/0x130
[  416.017031]  el0_svc_handler+0x38/0x78
[  416.020806]  el0_svc+0x8/0xc

Rework so that preemption is disabled when we loop over function
'BPF_PROG_RUN(...)'.

Fixes: 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>