Btrfs: Fix wrong error handling code

gcc says "warning: comparison of unsigned expression >= 0 is always
true" because i is an unsigned long. And gcc is right this time.
Signed-off-by: NStefan Behrens <sbehrens@giantdisaster.de>

fs/btrfs/extent_io.c

@@ -4104,8 +4104,8 @@ struct extent_buffer *alloc_dummy_extent_buffer(u64 start, unsigned long len)
 
 	return eb;
 err:
-	for (i--; i >= 0; i--)
-		__free_page(eb->pages[i]);
+	for (; i > 0; i--)
+		__free_page(eb->pages[i - 1]);
 	__free_extent_buffer(eb);
 	return NULL;
 }