powerpc/bpf: Fix use of user_pt_regs in uapi

Trying to build a .c file that includes <linux/bpf_perf_event.h>:
  $ cat test_bpf_headers.c
  #include <linux/bpf_perf_event.h>

throws the below error:
  /usr/include/linux/bpf_perf_event.h:14:28: error: field ‘regs’ has incomplete type
     14 |         bpf_user_pt_regs_t regs;
	|                            ^~~~

This is because we typedef bpf_user_pt_regs_t to 'struct user_pt_regs'
in arch/powerpc/include/uaps/asm/bpf_perf_event.h, but 'struct
user_pt_regs' is not exposed to userspace.

Powerpc has both pt_regs and user_pt_regs structures. However, unlike
arm64 and s390, we expose user_pt_regs to userspace as just 'pt_regs'.
As such, we should typedef bpf_user_pt_regs_t to 'struct pt_regs' for
userspace.

Within the kernel though, we want to typedef bpf_user_pt_regs_t to
'struct user_pt_regs'.

Remove arch/powerpc/include/uapi/asm/bpf_perf_event.h so that the
uapi/asm-generic version of the header is exposed to userspace.
Introduce arch/powerpc/include/asm/bpf_perf_event.h so that we can
typedef bpf_user_pt_regs_t to 'struct user_pt_regs' for use within the
kernel.

Note that this was not showing up with the bpf selftest build since
tools/include/uapi/asm/bpf_perf_event.h didn't include the powerpc
variant.

Fixes: a6460b03 ("powerpc/bpf: Fix broken uapi for BPF_PROG_TYPE_PERF_EVENT")
Cc: stable@vger.kernel.org # v4.20+
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
[mpe: Use typical naming for header include guard]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627191119.142867-1-naveen.n.rao@linux.vnet.ibm.com