Disable unused pathname::pn_path* (unneeded in Linux)

struct pathname is originally from Solaris VFS, and it has been used
in ZoL to merely call VOP from Linux VFS interface without API change,
therefore pathname::pn_path* are unused and unneeded. Technically,
struct pathname is a wrapper for C string in ZoL.

Saves stack a bit on lookup and unlink.

(#if0'd members instead of removing since comments refer to them.)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #9025

include/sys/pathname.h

@@ -54,8 +54,10 @@ 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;