bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack()

In the Linux kernel, the following vulnerability has been resolved: bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack() task_pt_regs() can return NULL on powerpc for kernel threads. This is then used in __bpf_get_stack() to check for user mode, resulting in a kernel oops. Guard against this by checking return value of task_pt_regs() before trying to obtain the call chain.