seqlock: Introduce scoped_seqlock_read()

seqlock: Introduce scoped_seqlock_read()

The read_seqbegin/need_seqretry/done_seqretry API is cumbersome and
error prone. With the new helper the "typical" code like

	int seq, nextseq;
	unsigned long flags;

	nextseq = 0;
	do {
		seq = nextseq;
		flags = read_seqbegin_or_lock_irqsave(&seqlock, &seq);

		// read-side critical section

		nextseq = 1;
	} while (need_seqretry(&seqlock, seq));
	done_seqretry_irqrestore(&seqlock, seq, flags);

can be rewritten as

	scoped_seqlock_read (&seqlock, ss_lock_irqsave) {
		// read-side critical section
	}

Original idea by Oleg Nesterov; with contributions from Linus.

Originally-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>