Unbreak build on Linux kernel < 3.10

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