bpf, riscv: Fix tail call count off by one in RV32 BPF JIT

This patch fixes an off by one error in the RV32 JIT handling for BPF
tail call. Currently, the code decrements TCC before checking if it
is less than zero. This limits the maximum number of tail calls to 32
instead of 33 as in other JITs. The fix is to instead check the old
value of TCC before decrementing.

Fixes: 5f316b65 ("riscv, bpf: Add RV32G eBPF JIT")
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Xi Wang <xi.wang@gmail.com>
Link: https://lore.kernel.org/bpf/20200421002804.5118-1-luke.r.nels@gmail.com

arch/riscv/net/bpf_jit_comp32.c

@@ -770,12 +770,13 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	emit_bcc(BPF_JGE, lo(idx_reg), RV_REG_T1, off, ctx);
 
 	/*
-	 * if ((temp_tcc = tcc - 1) < 0)
+	 * temp_tcc = tcc - 1;
+	 * if (tcc < 0)
 	 *   goto out;
 	 */
 	emit(rv_addi(RV_REG_T1, RV_REG_TCC, -1), ctx);
 	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
-	emit_bcc(BPF_JSLT, RV_REG_T1, RV_REG_ZERO, off, ctx);
+	emit_bcc(BPF_JSLT, RV_REG_TCC, RV_REG_ZERO, off, ctx);
 
 	/*
 	 * prog = array->ptrs[index];