powerpc/kprobes: Remove kretprobe_trampoline_holder.

Fixes the following testsuite failure:

  $ sudo ./perf test -v kallsyms
   1: vmlinux symtab matches kallsyms                          :
  --- start ---
  test child forked, pid 12489
  Using /proc/kcore for kernel object code
  Looking at the vmlinux_path (8 entries long)
  Using /boot/vmlinux for symbols
  0xc00000000003d300: diff name v: .kretprobe_trampoline_holder k: kretprobe_trampoline
  Maps only in vmlinux:
   c00000000086ca38-c000000000879b6c 87ca38 [kernel].text.unlikely
   c000000000879b6c-c000000000bf0000 889b6c [kernel].meminit.text
   c000000000bf0000-c000000000c53264 c00000 [kernel].init.text
   c000000000c53264-d000000004250000 c63264 [kernel].exit.text
   d000000004250000-d000000004450000 0 [libcrc32c]
   d000000004450000-d000000004620000 0 [xfs]
   d000000004620000-d000000004680000 0 [autofs4]
   d000000004680000-d0000000046e0000 0 [x_tables]
   d0000000046e0000-d000000004780000 0 [ip_tables]
   d000000004780000-d0000000047e0000 0 [rng_core]
   d0000000047e0000-ffffffffffffffff 0 [pseries_rng]
  Maps in vmlinux with a different name in kallsyms:
  Maps only in kallsyms:
   d000000000000000-f000000000000000 1000000000010000 [kernel.kallsyms]
   f000000000000000-ffffffffffffffff 3000000000010000 [kernel.kallsyms]
  test child finished with -1
  ---- end ----
  vmlinux symtab matches kallsyms: FAILED!

The problem is that the kretprobe_trampoline symbol looks like this:

  $ eu-readelf -s /boot/vmlinux G kretprobe_trampoline
   2431: c000000001302368     24 NOTYPE  LOCAL  DEFAULT       37 kretprobe_trampoline_holder
   2432: c00000000003d300      8 FUNC    LOCAL  DEFAULT        1 .kretprobe_trampoline_holder
  97543: c00000000003d300      0 NOTYPE  GLOBAL DEFAULT        1 kretprobe_trampoline

Its type is NOTYPE, and its size is 0, and this is a problem because
symbol-elf.c:dso__load_sym skips function symbols that are not STT_FUNC
or STT_GNU_IFUNC (this is determined by elf_sym__is_function). Even
if the type is changed to STT_FUNC, when dso__load_sym calls
symbols__fixup_duplicate, the kretprobe_trampoline symbol is dropped in
favour of .kretprobe_trampoline_holder because the latter has non-zero
size (as determined by choose_best_symbol).

With this patch, all vmlinux symbols match /proc/kallsyms and the
testcase passes.

Commit c1c355ce14c0 ("x86/kprobes: Get rid of
kretprobe_trampoline_holder()") gets rid of kretprobe_trampoline_holder
altogether on x86. This commit does the same on powerpc. This change
introduces no regressions on the perf and ftracetest testsuite results.

Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/kprobes.c

@@ -278,12 +278,11 @@ no_kprobe:
  * 	- When the probed function returns, this probe
  * 		causes the handlers to fire
  */
-static void __used kretprobe_trampoline_holder(void)
-{
-	asm volatile(".global kretprobe_trampoline\n"
-			"kretprobe_trampoline:\n"
-			"nop\n");
-}
+asm(".global kretprobe_trampoline\n"
+	".type kretprobe_trampoline, @function\n"
+	"kretprobe_trampoline:\n"
+	"nop\n"
+	".size kretprobe_trampoline, .-kretprobe_trampoline\n");
 
 /*
  * Called when the probe at kretprobe trampoline is hit