scsi: storvsc: Avoid excessive host scan on controller change

When there are multiple disks attached to the same SCSI controller, the
host may send several VSTOR_OPERATION_REMOVE_DEVICE or
VSTOR_OPERATION_ENUMERATE_BUS messages in a row, to indicate there is a
change on the SCSI controller. In response, storvsc rescans the SCSI
host.

There is no need to do multiple scans on the same host. Fix the code to
do only one scan.

[mkp: applied by hand]

Signed-off-by: Long Li <longli@microsoft.com>
Tested-by: Cathy Avery <cavery@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>