mm/slub: refactor calculate_order() and calc_slab_order()

After the previous cleanups, we can now move some code from
calc_slab_order() to calculate_order() so it's executed just once, and
do some more cleanups.

- move the min_order and MAX_OBJS_PER_PAGE evaluation to
  calc_slab_order().

- change calc_slab_order() parameter min_objects to min_order

Also make MAX_OBJS_PER_PAGE check more robust by considering also
min_objects in addition to slub_min_order. Otherwise this is not a
functional change.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>