rpmsg: virtio_rpmsg_bus: fix sg_set_buf() when addr is not a valid kernel address

To specify memory for remoteproc, we declare (dma_declare_coherent_memory())
an area which is ioremap'ed to the vmalloc area.  However, this address is
not a kernel address so virt_addr_valid(buf) fails.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Tested-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>