scsi: storvsc: Introduce the per-storvsc_device spinlock

scsi: storvsc: Introduce the per-storvsc_device spinlock

storvsc uses the spinlock of the hv_device's primary channel to
serialize modifications of stor_chns[] performed by storvsc_do_io()
and storvsc_change_target_cpu(), when it could/should use a (per-)
storvsc_device spinlock: this change untangles the synchronization
mechanism for the (storvsc-specific) stor_chns[] array from the
"generic" VMBus code and data structures, clarifying the scope of
this synchronization mechanism.

Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20200617164642.37393-8-parri.andrea@gmail.com
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>