arm64: Add get_user() type annotation on the !access_ok() path

Sparse reports "Using plain integer as NULL pointer" when the arm64
__get_user_error() assigns 0 to a pointer type. Use proper type
annotation.

Signed-of-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: kbuild test robot <lkp@intel.com>
Link: http://lkml.kernel.org/r/20200522142321.GP23230@ZenIV.linux.org.uk
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Al Viro

arch/arm64/include/asm/uaccess.h

@@ -304,7 +304,7 @@ do {									\
 		__p = uaccess_mask_ptr(__p);				\
 		__raw_get_user((x), __p, (err));			\
 	} else {							\
-		(x) = 0; (err) = -EFAULT;				\
+		(x) = (__force __typeof__(x))0; (err) = -EFAULT;	\
 	}								\
 } while (0)