Replace ZFS on Linux references with OpenZFS

This change updates the documentation to refer to the project
as OpenZFS instead ZFS on Linux.  Web links have been updated
to refer to https://github.com/openzfs/zfs.  The extraneous
zfsonlinux.org web links in the ZED and SPL sources have been
dropped.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11007

include/sys/pathname.h

@@ -54,10 +54,6 @@ extern "C" {
  */
 typedef struct pathname {
 	char	*pn_buf;		/* underlying storage */
-#if 0 /* unused in ZoL */
-	char	*pn_path;		/* remaining pathname */
-	size_t	pn_pathlen;		/* remaining length */
-#endif
 	size_t	pn_bufsize;		/* total size of pn_buf */
 } pathname_t;