fix block device size update serialization

[Christoph Hellwig]

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.