BugLink: http://bugs.launchpad.net/bugs/1670518

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>
(cherry picked from commit 0de8ce3e)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Marcelo Cerri <marcelo.cerri@canonical.com>
Acked-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Signed-off-by: Brad Figg <brad.figg@canonical.com>