[SCSI] fix scsi_reap_target() device_del from atomic context

scsi_reap_target() was desgined to be called from any context.
However it must do a device_del() of the target device, which may only
be called from user context.  Thus we have to reimplement
scsi_reap_target() via a workqueue.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>