mm/slub: simplify the last resort slab order calculation

If calculate_order() can't fit even a single large object within
slub_max_order, it will try using the smallest necessary order that may
exceed slub_max_order but not MAX_ORDER.

Currently this is done with a call to calc_slab_order() which is
unnecessary. We can simply use get_order(size). No functional change.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Feng Tang <feng.tang@intel.com>
Reviewed-and-tested-by: Jay Patel <jaypatel@linux.ibm.com>