fix block device size update serialization v2

[Christoph Hellwig]

Jens, can you consider this for 5.9?  It reliably fixes the reported
hangs with nvme hotremoval that we've had for a few releases.

On Sun, Aug 23, 2020 at 11:10:40AM +0200, Christoph Hellwig wrote:
> Hi Jens,
> 
> this series fixes how we update i_size for the block device inodes (and
> thus the block device).  Different helpers use two different locks
> (bd_mutex and i_rwsem) to protect the update, and it appears device
> mapper uses yet another internal lock.  A lot of the drivers do the
> update handcrafted in often crufty ways.  And in addition to that mess
> it turns out that the "main" lock, bd_mutex is pretty dead lock prone
> vs other spots in the block layer that acquire it during revalidation
> operations, as reported by Xianting.
> 
> Fix all that by adding a dedicated spinlock just for the size updates.
> 
> Changes since v1:
>  - don't call __invalidate_device under the new spinlock
>  - don't call into the file system code from the nvme removal code
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