bpf: arm64: Silence "UBSAN: negation-overflow" warning

With UBSAN, test_bpf.ko triggers warnings like:

UBSAN: negation-overflow in arch/arm64/net/bpf_jit_comp.c:1333:28
negation of -2147483648 cannot be represented in type 's32' (aka 'int'):

Silence these warnings by casting imm to u32 first.

Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Song Liu <song@kernel.org>
Tested-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20250218080240.2431257-1-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/arm64/net/bpf_jit_comp.c

@@ -272,7 +272,7 @@ static inline void emit_a64_add_i(const bool is64, const int dst, const int src,
 {
 	if (is_addsub_imm(imm)) {
 		emit(A64_ADD_I(is64, dst, src, imm), ctx);
-	} else if (is_addsub_imm(-imm)) {
+	} else if (is_addsub_imm(-(u32)imm)) {
 		emit(A64_SUB_I(is64, dst, src, -imm), ctx);
 	} else {
 		emit_a64_mov_i(is64, tmp, imm, ctx);
@@ -1159,7 +1159,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
 	case BPF_ALU64 | BPF_SUB | BPF_K:
 		if (is_addsub_imm(imm)) {
 			emit(A64_SUB_I(is64, dst, dst, imm), ctx);
-		} else if (is_addsub_imm(-imm)) {
+		} else if (is_addsub_imm(-(u32)imm)) {
 			emit(A64_ADD_I(is64, dst, dst, -imm), ctx);
 		} else {
 			emit_a64_mov_i(is64, tmp, imm, ctx);
@@ -1330,7 +1330,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
 	case BPF_JMP32 | BPF_JSLE | BPF_K:
 		if (is_addsub_imm(imm)) {
 			emit(A64_CMP_I(is64, dst, imm), ctx);
-		} else if (is_addsub_imm(-imm)) {
+		} else if (is_addsub_imm(-(u32)imm)) {
 			emit(A64_CMN_I(is64, dst, -imm), ctx);
 		} else {
 			emit_a64_mov_i(is64, tmp, imm, ctx);