powerpc/mm: Allow user space to map rtas_rmo_buf

With commit 90a545e9 (restrict /dev/mem to idle io memory ranges) mapping
rtas_rmo_buf from user space is failing. Hence we are not able to make
RTAS syscall.

This patch calls page_is_rtas_user_buf before calling iomem_is_exclusive
in  devmem_is_allowed(). This will allow user space to map rtas_rmo_buf
and we are able to make RTAS syscall.
Reported-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
CC: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/mem.c

@@ -560,12 +560,12 @@ subsys_initcall(add_system_ram_resources);
  */
 int devmem_is_allowed(unsigned long pfn)
 {
+	if (page_is_rtas_user_buf(pfn))
+		return 1;
 	if (iomem_is_exclusive(PFN_PHYS(pfn)))
 		return 0;
 	if (!page_is_ram(pfn))
 		return 1;
-	if (page_is_rtas_user_buf(pfn))
-		return 1;
 	return 0;
 }
 #endif /* CONFIG_STRICT_DEVMEM */