LoongArch: BPF: Fix sign extension for 12-bit immediates

When loading immediate values that fit within 12-bit signed range,
the move_imm function incorrectly used zero extension instead of
sign extension.

The bug was exposed when scx_simple scheduler failed with -EINVAL
in ops.init() after passing node = -1 to scx_bpf_create_dsq().
Due to incorrect sign extension, `node >= (int)nr_node_ids`
evaluated to true instead of false, causing BPF program failure.

Verified by testing with the scx_simple scheduler (located in
tools/sched_ext/). After building with `make` and running
./tools/sched_ext/build/bin/scx_simple, the scheduler now
initializes successfully with this fix.

Fix this by using sign extension (sext) instead of zero extension
for signed immediate values in move_imm.

Fixes: 5dc6155 ("LoongArch: Add BPF JIT support")
Reported-by: Bing Huang <[email protected]>
Signed-off-by: George Guo <[email protected]>

Author: georgejguo

arch/loongarch/net/bpf_jit.h

@@ -122,7 +122,8 @@ static inline void move_imm(struct jit_ctx *ctx, enum loongarch_gpr rd, long imm
 	/* addiw rd, $zero, imm_11_0 */
 	if (is_signed_imm12(imm)) {
 		emit_insn(ctx, addiw, rd, LOONGARCH_GPR_ZERO, imm);
-		goto zext;
+		emit_sext_32(ctx, rd, is32);
+		return;
 	}
 
 	/* ori rd, $zero, imm_11_0 */