scsi: mpt3sas: wait for and flush running commands on shutdown/unload

This patch finishes all outstanding SCSI IO commands (but not other commands,
e.g., task management) in the shutdown and unload paths.

It first waits for the commands to complete (this is done after setting
'ioc->remove_host = 1 ', which prevents new commands to be queued) then it
flushes commands that might still be running.

This avoids triggering error handling (e.g., abort command) for all commands
possibly completed by the adapter after interrupts disabled.

[mauricfo: introduced something in commit message.]

Signed-off-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Tested-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>