md related oops triggered in bdev_inode_switch_bdi

[NeilBrown]

Hi Christoph et. al.,

 My testing recently triggered an oops in bdi_lock_two called from
 bdev_inode_switch_bdi.
 The bdi and the request_queue that contains it had been freed.

 This happens with md which can free the md device and request queue
 immediately after last close.

 It seems that this is caused by your patch f758eeabeb96f8.
 Prior to that the 'old' bdi was never dereferenced in
 bdev_inode_switch_bdi.  Now it is.

 I think we can fix that by simply moving the call to bdev_inode_switch_bdi
 before the call to ->release as in the patch below.

 Do you see any problem with this patch?

Thanks,
NeilBrown