powerpc64/bpf: Do not save/restore LR on each call to bpf_stf_barrier()

Instead of saving and restoring LR before each invocation to
bpf_stf_barrier(), set SEEN_FUNC flag so that we save/restore LR in
prologue/epilogue.
Signed-off-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/4446f25478d82a2a4ac9dab2ebdfd88ddf923eb7.1644834730.git.naveen.n.rao@linux.vnet.ibm.com

arch/powerpc/net/bpf_jit_comp64.c

@@ -690,11 +690,10 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 				EMIT(PPC_RAW_ORI(_R31, _R31, 0));
 				break;
 			case STF_BARRIER_FALLBACK:
-				EMIT(PPC_RAW_MFLR(b2p[TMP_REG_1]));
+				ctx->seen |= SEEN_FUNC;
 				PPC_LI64(12, dereference_kernel_function_descriptor(bpf_stf_barrier));
 				EMIT(PPC_RAW_MTCTR(12));
 				EMIT(PPC_RAW_BCTRL());
-				EMIT(PPC_RAW_MTLR(b2p[TMP_REG_1]));
 				break;
 			case STF_BARRIER_NONE:
 				break;