Staging: hv: storvsc: Use mempools to allocate struct storvsc_cmd_request

We intend to use the storage driver to manage the root device.
To avoid deadlocks, use mempools to allocate struct storvsc_cmd_request.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>