When performing a firmware upgrade via atacontrol[1] the subsequent
command may time out producing the error message above. When this
happens the callout could still be active, and the system would then
panic due to a destroyed semaphore.
Instead, ensure that the callout is done first, via callout_drain.
Note that this fix applies to the "old" ata(4) and so isn't applicable
to the default configuration in HEAD. It is still applicable to
stable/8.
[1] http://lists.freebsd.org/pipermail/freebsd-current/2012-January/031122.html
Submitted by: Nima Misaghian
Reviewed by: rstone, attilio, mav
Obtained from: SVOS
MFC after: 3 days
1707b4455f