mm/slab: make __free(kfree) accept error pointers

commit cd7eb8f upstream.

Currently, if an automatically freed allocation is an error pointer that
will lead to a crash.  An example of this is in wm831x_gpio_dbg_show().

   171	char *label __free(kfree) = gpiochip_dup_line_label(chip, i);
   172	if (IS_ERR(label)) {
   173		dev_err(wm831x->dev, "Failed to duplicate label\n");
   174		continue;
   175  }

The auto clean up function should check for error pointers as well,
otherwise we're going to keep hitting issues like this.

Fixes: 54da6a0 ("locking: Introduce __cleanup() based infrastructure")
Cc: <stable@vger.kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/slab.h

@@ -228,7 +228,7 @@ void kfree(const void *objp);
 void kfree_sensitive(const void *objp);
 size_t __ksize(const void *objp);
 
-DEFINE_FREE(kfree, void *, if (_T) kfree(_T))
+DEFINE_FREE(kfree, void *, if (!IS_ERR_OR_NULL(_T)) kfree(_T))
 
 /**
  * ksize - Report actual allocation size of associated object