slab: remove cpu (partial) slabs usage from allocation paths

We now rely on sheaves as the percpu caching layer and can refill them
directly from partial or newly allocated slabs. Start removing the cpu
(partial) slabs code, first from allocation paths.

This means that any allocation not satisfied from percpu sheaves will
end up in ___slab_alloc(), where we remove the usage of cpu (partial)
slabs, so it will only perform get_partial() or new_slab(). In the
latter case we reuse alloc_from_new_slab() (when we don't use
the debug/tiny alloc_single_from_new_slab() variant).

In get_partial_node() we used to return a slab for freezing as the cpu
slab and to refill the partial slab. Now we only want to return a single
object and leave the slab on the list (unless it became full). We can't
simply reuse alloc_single_from_partial() as that assumes freeing uses
free_to_partial_list(). Instead we need to use __slab_update_freelist()
to work properly against a racing __slab_free().

To reflect the new purpose of get_partial() functions, rename them to
get_from_partial(), get_from_partial_node(), and get_from_any_partial().

The rest of the changes is removing functions that no longer have any
callers.

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Hao Li <hao.li@linux.dev>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>