block: fix use-after-free on cached last_lookup partition

hulk inclusion
category: bugfix
bugzilla: 27962
CVE: NA
---------------------------

delete_partition() clears the cached last_lookup partition. However
the .last_lookup cache may be overwritten by one IO path after
it is cleared from delete_partition(). Then another IO path may
use the cached deleting partition after __delete_partition() is
called, then use-after-free is triggered on the cached partition.

Fixes the issue by the following approach:

1) always get the partition's refcount via hd_struct_try_get() before
setting .last_lookup

2) move clearing .last_lookup from delete_partition() to
__delete_partition() which is release handle of the partition's
percpu-refcount, so that no IO path can overwrite .last_lookup after it
is cleared in __delete_partition().

It is one candidate approach of Yufen's patch[1] which adds overhead
in fast path by indirect lookup which may introduce one extra cacheline
in IO path. Also this patch relies on percpu-refcount's protection, and
it is easier to understand and verify.

[1] https://lore.kernel.org/linux-block/20200109013551.GB9655@ming.t460p/T/#t



Reported-by: Yufen Yu <yuyufen@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Hou Tao <houtao1@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Conflict:
	include/linux/genhd.h
	block/blk-core.c
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>