Batch up MODE_SELECT in rdac device handler.

LSI RDAC storage has the capability of handling mode selects for
multiple luns in a same command. Make use of that ability to send
as few MODE SELECTs as possible to the storage controller as possible.

This patch creates a work queue and queues up activate requests
when a MODE SELECT is sent down the wire. When that MODE SELECT
completes, it compiles queued up activate requests for multiple
luns into a single MODE SELECT.

This reduces the time to do failover/failback of large number of LUNS.
Signed-off-by: Babu Moger <babu.moger@lsi.com>
Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>