w1: fixup search to support abort from netlink

Before 63706172f3 "rework kthread_stop()" kthread_should_stop()
always returned false when called from a non-kthread task, after it
would oops as a non-kthread didn't have that structure and netlink was
calling search from a thread which wasn't a kthread.  9d1817cab2
"w1: fix oops when w1_search is called from netlink connector",
modified the code to avoid calling kthread_stop from a netlink thread.

Introduce a w1_master flag and bit W1_ABORT_SEARCH to identify abort
to cleanly support both kthread and netlink search abort.  A search
can take seconds to run, so it is important to abort early if the
hardware is removed in the middle of a search.

Signed-off-by: David Fries <David@Fries.net>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Cc: Marcin Jurkowski <marcin1j@gmail.com>
Cc: Josh Boyer <jwboyer@gmail.com>
Cc: Sven Geggus <lists@fuchsschwanzdomain.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/w1/w1.h

@@ -155,6 +155,14 @@ struct w1_bus_master
 		u8, w1_slave_found_callback);
 };
 
+/**
+ * enum w1_master_flags - bitfields used in w1_master.flags
+ * @W1_ABORT_SEARCH: abort searching early on shutdown
+ */
+enum w1_master_flags {
+	W1_ABORT_SEARCH = 0,
+};
+
 struct w1_master
 {
 	struct list_head	w1_master_entry;
@@ -178,6 +186,8 @@ struct w1_master
 	/** 5V strong pullup duration in milliseconds, zero disabled. */
 	int			pullup_duration;
 
+	long			flags;
+
 	struct task_struct	*thread;
 	struct mutex		mutex;
 	struct mutex		bus_mutex;