printk: introduce a kmsg_dump iterator

Rather than storing the iterator information in the registered
kmsg_dumper structure, create a separate iterator structure. The
kmsg_dump_iter structure can reside on the stack of the caller, thus
allowing lockless use of the kmsg_dump functions.

Update code that accesses the kernel logs using the kmsg_dumper
structure to use the new kmsg_dump_iter structure. For kmsg_dumpers,
this also means adding a call to kmsg_dump_rewind() to initialize
the iterator.

All this is in preparation for removal of @logbuf_lock.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org> # pstore
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210303101528.29901-13-john.ogness@linutronix.de

arch/powerpc/xmon/xmon.c

@@ -3005,7 +3005,7 @@ print_address(unsigned long addr)
 static void
 dump_log_buf(void)
 {
-	struct kmsg_dumper dumper;
+	struct kmsg_dump_iter iter;
 	unsigned char buf[128];
 	size_t len;
 
@@ -3017,9 +3017,9 @@ dump_log_buf(void)
 	catch_memory_errors = 1;
 	sync();
 
-	kmsg_dump_rewind_nolock(&dumper);
+	kmsg_dump_rewind_nolock(&iter);
 	xmon_start_pagination();
-	while (kmsg_dump_get_line_nolock(&dumper, false, buf, sizeof(buf), &len)) {
+	while (kmsg_dump_get_line_nolock(&iter, false, buf, sizeof(buf), &len)) {
 		buf[len] = '\0';
 		printf("%s", buf);
 	}