Drivers: hv: vmbus: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/hv/vmbus_drv.c:1132:31: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20220125180131.GA67746@embeddedor


Signed-off-by: Wei Liu <wei.liu@kernel.org>