block: move the part_stat* helpers from genhd.h to a new header

These macros are just used by a few files.  Move them out of genhd.h,
which is included everywhere into a new standalone header.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 65a7a43..d398b2d 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -24,6 +24,7 @@
 #include <linux/sysfs.h>
 #include <linux/quota.h>
 #include <linux/unicode.h>
+#include <linux/part_stat.h>
 
 #include "f2fs.h"
 #include "node.h"