mm, slub: don't aggressively inline with CONFIG_SLUB_TINY

SLUB fastpaths use __always_inline to avoid function calls. With
CONFIG_SLUB_TINY we would rather save the memory. Add a
__fastpath_inline macro that's __always_inline normally but empty with
CONFIG_SLUB_TINY.

bloat-o-meter results on x86_64 mm/slub.o:

add/remove: 3/1 grow/shrink: 1/8 up/down: 865/-1784 (-919)
Function                                     old     new   delta
kmem_cache_free                               20     281    +261
slab_alloc_node.isra                           -     245    +245
slab_free.constprop.isra                       -     231    +231
__kmem_cache_alloc_lru.isra                    -     128    +128
__kmem_cache_release                          88      83      -5
__kmem_cache_create                         1446    1436     -10
__kmem_cache_free                            271     142    -129
kmem_cache_alloc_node                        330     127    -203
kmem_cache_free_bulk.part                    826     613    -213
__kmem_cache_alloc_node                      230      10    -220
kmem_cache_alloc_lru                         325      12    -313
kmem_cache_alloc                             325      10    -315
kmem_cache_free.part                         376       -    -376
Total: Before=26103, After=25184, chg -3.52%

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>