riscv: fix VMALLOC_START definition

When below config items are set, compiler complained:

--------------------
CONFIG_CRASH_CORE=y
CONFIG_KEXEC_CORE=y
CONFIG_CRASH_DUMP=y
......
-----------------------

-------------------------------------------------------------------
arch/riscv/kernel/crash_core.c: In function 'arch_crash_save_vmcoreinfo':
arch/riscv/kernel/crash_core.c:11:58: warning: format '%lx' expects argument of type 'long unsigned int', but argument 2 has type 'int' [-Wformat=]
11 |         vmcoreinfo_append_str("NUMBER(VMALLOC_START)=0x%lx\n", VMALLOC_START);
   |                                                        ~~^
   |                                                          |
   |                                                          long unsigned int
   |                                                        %x
----------------------------------------------------------------------

This is because on riscv macro VMALLOC_START has different type when
CONFIG_MMU is set or unset.

arch/riscv/include/asm/pgtable.h:
--------------------------------------------------

Changing it to _AC(0, UL) in case CONFIG_MMU=n can fix the warning.

Link: https://lkml.kernel.org/r/ZW7OsX4zQRA3mO4+@MiWiFi-R3L-srvSigned-off-by: Baoquan He <bhe@redhat.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>	# build-tested
Cc: Eric DeVolder <eric_devolder@yahoo.com>
Cc: Ignat Korchagin <ignat@cloudflare.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/riscv/include/asm/pgtable.h

@@ -899,7 +899,7 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
 #define PAGE_KERNEL		__pgprot(0)
 #define swapper_pg_dir		NULL
 #define TASK_SIZE		0xffffffffUL
-#define VMALLOC_START		0
+#define VMALLOC_START		_AC(0, UL)
 #define VMALLOC_END		TASK_SIZE
 
 #endif /* !CONFIG_MMU */