mm/fadvise.c: fix signed overflow UBSAN complaint

Signed integer overflow is undefined according to the C standard.  The
overflow in ksys_fadvise64_64() is deliberate, but since it is signed
overflow, UBSAN complains:

	UBSAN: Undefined behaviour in mm/fadvise.c:76:10
	signed integer overflow:
	4 + 9223372036854775805 cannot be represented in type 'long long int'

Use unsigned types to do math.  Unsigned overflow is defined so UBSAN
will not complain about it.  This patch doesn't change generated code.

[[email protected]: add comment explaining the casts]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrey Ryabinin <[email protected]>
Reported-by: <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/fadvise.c

@@ -72,8 +72,12 @@ int ksys_fadvise64_64(int fd, loff_t offset, loff_t len, int advice)
 		goto out;
 	}
 
-	/* Careful about overflows. Len == 0 means "as much as possible" */
-	endbyte = offset + len;
+	/*
+	 * Careful about overflows. Len == 0 means "as much as possible".  Use
+	 * unsigned math because signed overflows are undefined and UBSan
+	 * complains.
+	 */
+	endbyte = (u64)offset + (u64)len;
 	if (!len || endbyte < len)
 		endbyte = -1;
 	else