Drivers: hv: vmbus: Use cpumask_var_t for on-stack cpu mask

A cpumask structure on the stack can cause a warning with
CONFIG_NR_CPUS=8192 (e.g. Ubuntu 16.04 and 18.04 use this):

drivers/hv//channel_mgmt.c: In function ‘init_vp_index’:
drivers/hv//channel_mgmt.c:702:1: warning: the frame size of 1032 bytes
  is larger than 1024 bytes [-Wframe-larger-than=]

Nowadays it looks most distros enable CONFIG_CPUMASK_OFFSTACK=y, and
hence we can work around the warning by using cpumask_var_t.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>