powerpc/bpf: Fix detecting BPF atomic instructions

Commit 91c960b0 ("bpf: Rename BPF_XADD and prepare to encode other
atomics in .imm") converted BPF_XADD to BPF_ATOMIC and added a way to
distinguish instructions based on the immediate field. Existing JIT
implementations were updated to check for the immediate field and to
reject programs utilizing anything more than BPF_ADD (such as BPF_FETCH)
in the immediate field.

However, the check added to powerpc64 JIT did not look at the correct
BPF instruction. Due to this, such programs would be accepted and
incorrectly JIT'ed resulting in soft lockups, as seen with the atomic
bounds test. Fix this by looking at the correct immediate value.

Fixes: 91c960b0 ("bpf: Rename BPF_XADD and prepare to encode other atomics in .imm")
Reported-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4117b430ffaa8cd7af042496f87fd7539e4f17fd.1625145429.git.naveen.n.rao@linux.vnet.ibm.com

arch/powerpc/net/bpf_jit_comp64.c

@@ -667,7 +667,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 		 * BPF_STX ATOMIC (atomic ops)
 		 */
 		case BPF_STX | BPF_ATOMIC | BPF_W:
-			if (insn->imm != BPF_ADD) {
+			if (imm != BPF_ADD) {
 				pr_err_ratelimited(
 					"eBPF filter atomic op code %02x (@%d) unsupported\n",
 					code, i);
@@ -689,7 +689,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 			PPC_BCC_SHORT(COND_NE, tmp_idx);
 			break;
 		case BPF_STX | BPF_ATOMIC | BPF_DW:
-			if (insn->imm != BPF_ADD) {
+			if (imm != BPF_ADD) {
 				pr_err_ratelimited(
 					"eBPF filter atomic op code %02x (@%d) unsupported\n",
 					code, i);