UBI: document UBI_IOCVOLUP better in user header

The current ioctl define implies that this func expects to be passed a
64bit number directly rather than a pointer to a 64bit.  The code that
processes this ioctl shows that it clearly expects a pointer.

It'd be best if we could change the type to "__s64*", but that would
change the generated ioctl number thus breaking the userland ABI.  So
just add a comment for intrepid developers.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

include/uapi/mtd/ubi-user.h

@@ -173,7 +173,10 @@
 
 #define UBI_VOL_IOC_MAGIC 'O'
 
-/* Start UBI volume update */
+/* Start UBI volume update
+ * Note: This actually takes a pointer (__s64*), but we can't change
+ *       that without breaking the ABI on 32bit systems
+ */
 #define UBI_IOCVOLUP _IOW(UBI_VOL_IOC_MAGIC, 0, __s64)
 /* LEB erasure command, used for debugging, disabled by default */
 #define UBI_IOCEBER _IOW(UBI_VOL_IOC_MAGIC, 1, __s32)