Unbreak build on Linux kernel < 3.10

d12614521a("Fixes for procfs files backed by linked lists")
uses PDE_DATA(), but since PDE_DATA() (public interface which
replaced old public interface PDE()) first appeared in upstream
kernel 3.10, it lacks visible local definition for kernel < 3.10.

Move the local PDE_DATA() definition to a ZoL header, to unbreak
build on kernel < 3.10.

--
module/spl/spl-procfs-list.c: In function 'procfs_list_open':
module/spl/spl-procfs-list.c:166: error: implicit declaration of function 'PDE_DATA'
module/spl/spl-procfs-list.c:166: warning: assignment makes pointer from integer without a cast

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Gallagher <john.gallagher@delphix.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8599

include/spl/sys/proc.h

@@ -27,6 +27,10 @@
 
 #include <linux/proc_fs.h>
 
+#ifndef HAVE_PDE_DATA
+#define	PDE_DATA(x) (PDE(x)->data)
+#endif
+
 extern struct proc_dir_entry *proc_spl_kstat;
 
 int spl_proc_init(void);

module/spl/spl-kstat.c

@@ -30,10 +30,6 @@
 #include <sys/cmn_err.h>
 #include <sys/sysmacros.h>
 
-#ifndef HAVE_PDE_DATA
-#define	PDE_DATA(x) (PDE(x)->data)
-#endif
-
 static kmutex_t kstat_module_lock;
 static struct list_head kstat_module_list;
 static kid_t kstat_id;