bpf: powerpc: fix broken uapi for BPF_PROG_TYPE_PERF_EVENT

Now that there are different variants of pt_regs for userspace and
kernel, the uapi for the BPF_PROG_TYPE_PERF_EVENT program type must
be changed by exporting the user_pt_regs structure instead of the
pt_regs structure that is in-kernel only.

Fixes: 002af939 ("powerpc: Split user/kernel definitions of struct pt_regs")
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>