FAQ: Add Q&A about kernel lockdown (#2532)

add kernel lockdown and solution to turn it off.

FAQ.txt

@@ -21,6 +21,15 @@ Q: hello_world.py still fails with:
  Exception: Failed to load BPF program hello
 A: sudo
 
+Q: hello_world.py fails with
+ bpf: Failed to load program: Operation not permitted
+ despite running as root, and strace shows each `bpf()` system call failing with an EPERM.
+A: The so-called Kernel lockdown might be the root cause. Try disabling it with the so-called
+ sysrq mechanism:
+ echo 1 > /proc/sys/kernel/sysrq
+ echo x > /proc/sysrq-trigger
+ Also see https://github.com/iovisor/bcc/issues/2525
+
 Q: How do I fulfill the Linux kernel version requirement?
 A: You need to obtain a recent version of the Linux source code
  (please look at the README for the exact version), enable the