Add pn_alloc()/pn_free() functions

In order to remove the HAVE_PN_UTILS wrappers the pn_alloc() and
pn_free() functions must be implemented.  The existing illumos
implementation were used for this purpose.

The `flags` argument which was used in places wrapped by the
HAVE_PN_UTILS condition has beed added back to zfs_remove() and
zfs_link() functions.  This removes a small point of divergence
between the ZoL code and upstream.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4522

include/sys/pathname.h

@@ -0,0 +1,70 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 1989, 2010, Oracle and/or its affiliates. All rights reserved.
+ */
+
+/*	Copyright (c) 1983, 1984, 1985, 1986, 1987, 1988, 1989 AT&T	*/
+/*	  All Rights Reserved  	*/
+
+/*
+ * Portions of this source code were derived from Berkeley 4.3 BSD
+ * under license from the Regents of the University of California.
+ */
+
+#ifndef _SYS_PATHNAME_H
+#define	_SYS_PATHNAME_H
+
+#ifdef	__cplusplus
+extern "C" {
+#endif
+
+/*
+ * Pathname structure.
+ * System calls that operate on path names gather the path name
+ * from the system call into this structure and reduce it by
+ * peeling off translated components.  If a symbolic link is
+ * encountered the new path name to be translated is also
+ * assembled in this structure.
+ *
+ * By convention pn_buf is not changed once it's been set to point
+ * to the underlying storage; routines which manipulate the pathname
+ * do so by changing pn_path and pn_pathlen.  pn_pathlen is redundant
+ * since the path name is null-terminated, but is provided to make
+ * some computations faster.
+ */
+typedef struct pathname {
+	char	*pn_buf;		/* underlying storage */
+	char	*pn_path;		/* remaining pathname */
+	size_t	pn_pathlen;		/* remaining length */
+	size_t	pn_bufsize;		/* total size of pn_buf */
+} pathname_t;
+
+extern void	pn_alloc(struct pathname *);
+extern void	pn_alloc_sz(struct pathname *, size_t);
+extern void	pn_free(struct pathname *);
+
+#ifdef	__cplusplus
+}
+#endif
+
+#endif	/* _SYS_PATHNAME_H */