riscv: Ensure BPF_JIT_REGION_START aligned with PMD size

Andreas reported commit fc850476 ("riscv: bpf: Avoid breaking W^X")
breaks booting with one kind of defconfig, I reproduced a kernel panic
with the defconfig:

[    0.138553] Unable to handle kernel paging request at virtual address ffffffff81201220
[    0.139159] Oops [#1]
[    0.139303] Modules linked in:
[    0.139601] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.13.0-rc5-default+ #1
[    0.139934] Hardware name: riscv-virtio,qemu (DT)
[    0.140193] epc : __memset+0xc4/0xfc
[    0.140416]  ra : skb_flow_dissector_init+0x1e/0x82
[    0.140609] epc : ffffffff8029806c ra : ffffffff8033be78 sp : ffffffe001647da0
[    0.140878]  gp : ffffffff81134b08 tp : ffffffe001654380 t0 : ffffffff81201158
[    0.141156]  t1 : 0000000000000002 t2 : 0000000000000154 s0 : ffffffe001647dd0
[    0.141424]  s1 : ffffffff80a43250 a0 : ffffffff81201220 a1 : 0000000000000000
[    0.141654]  a2 : 000000000000003c a3 : ffffffff81201258 a4 : 0000000000000064
[    0.14189...

Documentation/riscv/vm-layout.rst

@@ -58,6 +58,6 @@ RISC-V Linux Kernel SV39
                                                               |
   ____________________________________________________________|____________________________________________________________
                     |            |                  |         |
-   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules
-   ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel, BPF
+   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | modules, BPF
+   ffffffff80000000 |   -2    GB | ffffffffffffffff |    2 GB | kernel
   __________________|____________|__________________|_________|____________________________________________________________

arch/riscv/include/asm/pgtable.h

@@ -30,9 +30,8 @@
 
 #define BPF_JIT_REGION_SIZE	(SZ_128M)
 #ifdef CONFIG_64BIT
-/* KASLR should leave at least 128MB for BPF after the kernel */
-#define BPF_JIT_REGION_START	PFN_ALIGN((unsigned long)&_end)
-#define BPF_JIT_REGION_END	(BPF_JIT_REGION_START + BPF_JIT_REGION_SIZE)
+#define BPF_JIT_REGION_START	(BPF_JIT_REGION_END - BPF_JIT_REGION_SIZE)
+#define BPF_JIT_REGION_END	(MODULES_END)
 #else
 #define BPF_JIT_REGION_START	(PAGE_OFFSET - BPF_JIT_REGION_SIZE)
 #define BPF_JIT_REGION_END	(VMALLOC_END)

arch/riscv/mm/kasan_init.c

@@ -201,7 +201,7 @@ void __init kasan_init(void)
 
 	/* Populate kernel, BPF, modules mapping */
 	kasan_populate(kasan_mem_to_shadow((const void *)MODULES_VADDR),
-		       kasan_mem_to_shadow((const void *)BPF_JIT_REGION_END));
+		       kasan_mem_to_shadow((const void *)MODULES_VADDR + SZ_2G));
 
 	for (i = 0; i < PTRS_PER_PTE; i++)
 		set_pte(&kasan_early_shadow_pte[i],