libata: fix shutdown warning message printing

Unlocking ap->lock and ssleeping don't work because SCSI commands can
be issued from completion path without context.  Reimplement delayed
completion by allowing translation functions to override
qc->scsidone(), storing the original completion function to
scmd->scsi_done() and overriding qc->scsidone() with a function which
schedules delayed invocation of scmd->scsi_done().

This isn't pretty at all but all the ugly parts are thankfully
contained in the stop translation path where the compat feature is
implemented.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>