powerpc/64: indirect function call use bctrl rather than blrl in ret_from_kernel_thread

blrl is not recommended to use as an indirect function call, as it may
corrupt the link stack predictor.

This is not a performance critical path but this should be fixed for
consistency.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200611121119.1015740-1-npiggin@gmail.com

arch/powerpc/kernel/entry_64.S

@@ -233,12 +233,12 @@ _GLOBAL(ret_from_fork)
 _GLOBAL(ret_from_kernel_thread)
 	bl	schedule_tail
 	REST_NVGPRS(r1)
-	mtlr	r14
+	mtctr	r14
 	mr	r3,r15
 #ifdef PPC64_ELF_ABI_v2
 	mr	r12,r14
 #endif
-	blrl
+	bctrl
 	li	r3,0
 	b	.Lsyscall_exit