md: avoid signed overflow in slot_store()

slot_store() uses kstrtouint() to get a slot number, but stores the
result in an "int" variable (by casting a pointer).
This can result in a negative slot number if the unsigned int value is
very large.

A negative number means that the slot is empty, but setting a negative
slot number this way will not remove the device from the array.  I don't
think this is a serious problem, but it could cause confusion and it is
best to fix it.
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md.c

@@ -3128,6 +3128,9 @@ slot_store(struct md_rdev *rdev, const char *buf, size_t len)
 		err = kstrtouint(buf, 10, (unsigned int *)&slot);
 		if (err < 0)
 			return err;
+		if (slot < 0)
+			/* overflow */
+			return -ENOSPC;
 	}
 	if (rdev->mddev->pers && slot == -1) {
 		/* Setting 'slot' on an active array requires also