powerpc/entry32: Blacklist syscall exit points for kprobe.

kprobe does not handle events happening in real mode.

The very last part of syscall cannot support a trap.
Add a symbol syscall_exit_finish to identify that part and
blacklist it from kprobe.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/23eddf49abb03d1359fa0be4206998eb3800f42c.1585670437.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/entry_32.S

@@ -462,6 +462,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
 	lwz	r7,_NIP(r1)
 	lwz	r2,GPR2(r1)
 	lwz	r1,GPR1(r1)
+syscall_exit_finish:
 #if defined(CONFIG_PPC_8xx) && defined(CONFIG_PERF_EVENTS)
 	mtspr	SPRN_NRI, r0
 #endif
@@ -469,6 +470,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
 	mtspr	SPRN_SRR1,r8
 	SYNC
 	RFI
+_ASM_NOKPROBE_SYMBOL(syscall_exit_finish)
 #ifdef CONFIG_44x
 2:	li	r7,0
 	iccci	r0,r0
@@ -600,6 +602,7 @@ ret_from_kernel_syscall:
 	mtspr	SPRN_SRR1, r10
 	SYNC
 	RFI
+_ASM_NOKPROBE_SYMBOL(ret_from_kernel_syscall)
 
 /*
  * The fork/clone functions need to copy the full register set into