LoongArch: Fix MAX_REG_OFFSET calculation

Fix MAX_REG_OFFSET calculation, make it point to the last register
in 'struct pt_regs' and not to the marker itself, which could allow
regs_get_register() to return an invalid offset.

Cc: stable@vger.kernel.org
Fixes: 803b0fc ("LoongArch: Add process management")
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/ptrace.h

@@ -55,7 +55,7 @@ static inline void instruction_pointer_set(struct pt_regs *regs, unsigned long v
 
 /* Query offset/name of register from its name/offset */
 extern int regs_query_register_offset(const char *name);
-#define MAX_REG_OFFSET (offsetof(struct pt_regs, __last))
+#define MAX_REG_OFFSET (offsetof(struct pt_regs, __last) - sizeof(unsigned long))
 
 /**
  * regs_get_register() - get register value from its offset