w1: fixup search to support abort from netlink

Before 63706172 "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.  9d1817ca
"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>