device is not registered in sysfs.  Before async scan it was OK, because
before releasing scan_mutex old code called either scsi_sysfs_add_sdev()
or scsi_destroy_sdev() and, therefore, completed the work or discarded it.

With async scan the invariant is broken and scsi crashes in
__scsi_remove_device() when trying to unregister not registered devices.

The fix could be introducing new state(s), which is equivalent to
SDEV_RUNNING, except for one thing, we know that scsi_sysfs_add_sdev() has
not been called yet.  Or a separate flag, because the state can be
SDEV_BLOCK or even something else.

Simpler way is just to check that the device is regstered in sysfs before
unregistering.  Another operations in __scsi_remove_device() seem to be
idempotent or even required, because scsi_add_lun() makes some part of
work duplicated in scsi_sysfs_add_sdev().
Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>