riscv: Fix TASK_SIZE on 64-bit NOMMU

On NOMMU, userspace memory can come from anywhere in physical RAM. The
current definition of TASK_SIZE is wrong if any RAM exists above 4G,
causing spurious failures in the userspace access routines.

Fixes: 6bd33e1 ("riscv: add nommu support")
Fixes: c3f896d ("mm: switch the test_vmalloc module to use __vmalloc_node")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Bo Gan <ganboing@gmail.com>
Link: https://lore.kernel.org/r/20240227003630.3634533-2-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/pgtable.h

@@ -882,7 +882,7 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
 #define PAGE_SHARED		__pgprot(0)
 #define PAGE_KERNEL		__pgprot(0)
 #define swapper_pg_dir		NULL
-#define TASK_SIZE		0xffffffffUL
+#define TASK_SIZE		_AC(-1, UL)
 #define VMALLOC_START		_AC(0, UL)
 #define VMALLOC_END		TASK_SIZE