mm/slab_common: drop kmem_alloc & avoid dereferencing fields when not using

mm/slab_common: drop kmem_alloc & avoid dereferencing fields when not using

Drop kmem_alloc event class, and define kmalloc and kmem_cache_alloc
using TRACE_EVENT() macro.

And then this patch does:
   - Do not pass pointer to struct kmem_cache to trace_kmalloc.
     gfp flag is enough to know if it's accounted or not.
   - Avoid dereferencing s->object_size and s->size when not using kmem_cache_alloc event.
   - Avoid dereferencing s->name in when not using kmem_cache_free event.
   - Adjust s->size to SLOB_UNITS(s->size) * SLOB_UNIT in SLOB

Cc: Vasily Averin <[email protected]>
Suggested-by: Vlastimil Babka <[email protected]>
Signed-off-by: Hyeonggon Yoo <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>