Fixes for procfs files backed by linked lists

There are some issues with the way the seq_file interface is implemented
for kstats backed by linked lists (zfs_dbgmsgs and certain per-pool
debugging info):

* We don't account for the fact that seq_file sometimes visits a node
  multiple times, which results in missing messages when read through
  procfs.
* We don't keep separate state for each reader of a file, so concurrent
  readers will receive incorrect results.
* We don't account for the fact that entries may have been removed from
  the list between read syscalls, so reading from these files in procfs
  can cause the system to crash.

This change fixes these issues and adds procfs_list, a wrapper around a
linked list which abstracts away the details of implementing the
seq_file interface for a list and exposing the contents of the list
through procfs.

Reviewed by: Don Brady <don.brady@delphix.com>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Gallagher <john.gallagher@delphix.com>
External-issue: LX-1211
Closes #7819

include/sys/zfs_context.h

@@ -62,6 +62,7 @@
 #include <sys/ctype.h>
 #include <sys/disp.h>
 #include <sys/trace.h>
+#include <sys/procfs_list.h>
 #include <linux/dcache_compat.h>
 #include <linux/utsname_compat.h>
 
@@ -351,6 +352,37 @@ extern void kstat_set_raw_ops(kstat_t *ksp,
     int (*data)(char *buf, size_t size, void *data),
     void *(*addr)(kstat_t *ksp, loff_t index));
 
+/*
+ * procfs list manipulation
+ */
+
+struct seq_file { };
+void seq_printf(struct seq_file *m, const char *fmt, ...);
+
+typedef struct procfs_list {
+	void		*pl_private;
+	kmutex_t	pl_lock;
+	list_t		pl_list;
+	uint64_t	pl_next_id;
+	size_t		pl_node_offset;
+} procfs_list_t;
+
+typedef struct procfs_list_node {
+	list_node_t	pln_link;
+	uint64_t	pln_id;
+} procfs_list_node_t;
+
+void procfs_list_install(const char *module,
+    const char *name,
+    procfs_list_t *procfs_list,
+    int (*show)(struct seq_file *f, void *p),
+    int (*show_header)(struct seq_file *f),
+    int (*clear)(procfs_list_t *procfs_list),
+    size_t procfs_list_node_off);
+void procfs_list_uninstall(procfs_list_t *procfs_list);
+void procfs_list_destroy(procfs_list_t *procfs_list);
+void procfs_list_add(procfs_list_t *procfs_list, void *p);
+
 /*
  * Kernel memory
  */