nvlist: Add nvlist_snprintf() and zfs_dbgmsg_nvlist()

Add nvlist_snprintf() to print a nvlist to a buffer.  This is basically
the snprintf() version of dump_nvlist().  Along with that, add a
zfs_dbgmsg_nvlist() to print out an nvlist to dbgmsg.  This will aid in
debugging.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17215

module/zfs/zfs_debug_common.c

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * 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 https://opensource.org/licenses/CDDL-1.0.
+ * 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) 2025 by Lawrence Livermore National Security, LLC.
+ */
+/*
+ * This file contains zfs_dbgmsg() specific functions that are not OS or
+ * userspace specific.
+ */
+#if !defined(_KERNEL)
+#include <string.h>
+#endif
+
+#include <sys/zfs_context.h>
+#include <sys/zfs_debug.h>
+#include <sys/nvpair.h>
+
+/*
+ * Given a multi-line string, print out one of the lines and return a pointer
+ * to the next line. Lines are demarcated by '\n'.  Note: this modifies the
+ * input string (buf[]).
+ *
+ * This function is meant to be used in a loop like:
+ * 	while (buf != NULL)
+ * 		buf = kernel_print_one_line(buf);
+ *
+ * This function is useful for printing large, multi-line text buffers.
+ *
+ * Returns the pointer to the beginning of the next line in buf[], or NULL
+ * if it's the last line, or nothing more to print.
+ */
+static char *
+zfs_dbgmsg_one_line(char *buf)
+{
+	char *nl;
+	if (!buf)
+		return (NULL);
+
+	nl = strchr(buf, '\n');
+	if (nl == NULL) {
+		__zfs_dbgmsg(buf);
+		return (NULL); /* done */
+	}
+	*nl = '\0';
+	__zfs_dbgmsg(buf);
+
+	return (nl + 1);
+}
+
+/*
+ * Dump an nvlist tree to dbgmsg.
+ *
+ * This is the zfs_dbgmsg version of userspace's dump_nvlist() from libnvpair.
+ */
+void
+__zfs_dbgmsg_nvlist(nvlist_t *nv)
+{
+	int len;
+	char *buf;
+
+	len = nvlist_snprintf(NULL, 0, nv, 4);
+	len++; /* Add null terminator */
+
+	buf = vmem_alloc(len, KM_SLEEP);
+	if (buf == NULL)
+		return;
+
+	(void) nvlist_snprintf(buf, len, nv, 4);
+
+	while (buf != NULL)
+		buf = zfs_dbgmsg_one_line(buf);
+
+	vmem_free(buf, len);
+}
+
+#ifdef _KERNEL
+EXPORT_SYMBOL(__zfs_dbgmsg_nvlist);
+#endif