Linux 5.0 compat: Use totalram_pages()

totalram_pages() was converted to an atomic variable in 5.0:

https://patchwork.kernel.org/patch/10652795/

Its value should now be read though the totalram_pages() helper
function.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #8263

module/zfs/arc.c

@@ -4004,9 +4004,9 @@ arc_all_memory(void)
 {
 #ifdef _KERNEL
 #ifdef CONFIG_HIGHMEM
-	return (ptob(totalram_pages - totalhigh_pages));
+	return (ptob(zfs_totalram_pages - totalhigh_pages));
 #else
-	return (ptob(totalram_pages));
+	return (ptob(zfs_totalram_pages));
 #endif /* CONFIG_HIGHMEM */
 #else
 	return (ptob(physmem) / 2);