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.