[SCSI] aacraid: Fix for arrays are going offline in the system. System hangs

One of the customer had reported that the set of raid logical arrays will
become unavailable (I/O offline) after a long hours of IO stress test.  The OS
wouldn`t be accessible afterwards and require a hard reset.

This driver patch has a fix for race condition between the doorbell and the
circular buffer. The driver is modified to do an extra read after clearing the
doorbell in case there had been a completion posted during the small timing
window.

With this fix, we ran IO stress for ~13 days. There were no IO failures.

Signed-off-by: Mahesh Rajashekhara <Mahesh.Rajashekhara@pmcs.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>