With allocation fastpaths no longer divided between two .c files, we
have better inlining, however checking the disassembly of
kmem_cache_alloc() reveals we can do better to make the fastpaths
smaller and move the less common situations out of line or to separate
functions, to reduce instruction cache pressure.

- split memcg pre/post alloc hooks to inlined checks that use likely()
  to assume there will be no objcg handling necessary, and non-inline
  functions doing the actual handling

- add some more likely/unlikely() to pre/post alloc hooks to indicate
  which scenarios should be out of line

- change gfp_allowed_mask handling in slab_post_alloc_hook() so the
  code can be optimized away when kasan/kmsan/kmemleak is configured out

bloat-o-meter shows:
add/remove: 4/2 grow/shrink: 1/8 up/down: 521/-2924 (-2403)
Function                                     old     new   delta
__memcg_slab_post_alloc_hook                   -     461    +461
kmem_cache_alloc_bulk                        775     791     +16
__pfx_should_failslab.constprop                -      16     +16
__pfx___memcg_slab_post_alloc_hook             -      16     +16
should_failslab.constprop                      -      12     +12
__pfx_memcg_slab_post_alloc_hook              16       -     -16
kmem_cache_alloc_lru                        1295    1023    -272
kmem_cache_alloc_node                       1118     817    -301
kmem_cache_alloc                            1076     772    -304
kmalloc_node_trace                          1149     838    -311
kmalloc_trace                               1102     789    -313
__kmalloc_node_track_caller                 1393    1080    -313
__kmalloc_node                              1397    1082    -315
__kmalloc                                   1374    1059    -315
memcg_slab_post_alloc_hook                   464       -    -464

Note that gcc still decided to inline __memcg_pre_alloc_hook(), but the
code is out of line. Forcing noinline did not improve the results. As a
result the fastpaths are shorter and overal code size is reduced.
Acked-by: David Rientjes <rientjes@google.com>
Tested-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>