fix unbalanced page refcounting in bio_map_user_iov

bio_map_user_iov and bio_unmap_user do unbalanced pages refcounting if
IO vector has small consecutive buffers belonging to the same page.
bio_add_pc_page merges them into one, but the page reference is never
dropped.

Cc: stable@vger.kernel.org
Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
static inline bool bio_remaining_done(struct bio *bio)
{
	/*
	 * If we're not chaining, then ->__bi_remaining is always 1 and
	 * we always end io on the first invocation.
	 */
	if (!bio_flagged(bio, BIO_CHAIN))
		return true;

	BUG_ON(atomic_read(&bio->__bi_remaining) <= 0);

	if (atomic_dec_and_test(&bio->__bi_remaining)) {
		bio_clear_flag(bio, BIO_CHAIN);
		return true;
	}

	return false;
}

static inline bool bio_remaining_done(struct bio *bio)
{
	/*
	 * If we're not chaining, then ->__bi_remaining is always 1 and
	 * we always end io on the first invocation.
	 */
	if (!bio_flagged(bio, BIO_CHAIN))
		return true;

	BUG_ON(atomic_read(&bio->__bi_remaining) <= 0);

	if (atomic_dec_and_test(&bio->__bi_remaining)) {
		bio_clear_flag(bio, BIO_CHAIN);
		return true;
	}

	return false;
}
