x86/resctrl: Fix stale comment

The comment in rdtgroup_init() refers to the non existent function
rdt_mount(), which has now been renamed rdt_get_tree(). Fix the
comment.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lkml.kernel.org/r/20200708163929.2783-4-james.morse@arm.com