Introduce function _scsih_nvme_shutdown() to issue IO Unit Control message
to IOC firmware with operation code 'shutdown'. This causes IOC firmware to
issue NVMe shutdown commands to all NVMe drives attached to it.

NVMe Shutdown:

NVMe devices need to have a specific shutdown sequence performed before
power is removed. For this, the IOC firmware needs to be notified when the
system is being shutdown. So during the system shutdown time, driver issues
an IO Unit Control request with operation code MPI26_CTRL_OP_SHUTDOWN to
inform firmware that a shutdown is initiated.

This shutdown command is issued only if NVMe devices are attached to the
controller.

During each NVMe device addition, driver reads pcie device page2 to get
shutdown latency (e.g. drive's RTD3 Entry Latency) and updates the max
latency value among the added NVMe drives in ioc->max_shutdown_latency.
This is used as the timeout value for IO Unit Control command at the time
of shutdown.

When a NVMe drive is removed and its shutdown latency matches which
ioc->max_shutdown_latency then ioc->max_shutdown_latency is updated to next
max value (by iterating over the list of available devices).  If the
shutdown latency is 0, then default timeout is set to six seconds.

Link: https://lore.kernel.org/r/20191226111333.26131-3-sreekanth.reddy@broadcom.comSigned-off-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>