hv_do_hypercall() assumes that we pass a segment from a physically
contiguous buffer.  A buffer allocated on the stack may not work if
CONFIG_VMAP_STACK=y is set.

Use kmalloc() to allocate this buffer.
Reported-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>