slab: Introduce kmalloc_obj() and family

Introduce type-aware kmalloc-family helpers to replace the common
idioms for single object and arrays of objects allocation:

ptr = kmalloc(sizeof(*ptr), gfp);
ptr = kmalloc(sizeof(struct some_obj_name), gfp);
ptr = kzalloc(sizeof(*ptr), gfp);
ptr = kmalloc_array(count, sizeof(*ptr), gfp);
ptr = kcalloc(count, sizeof(*ptr), gfp);

These become, respectively:

ptr = kmalloc_obj(*ptr, gfp);
ptr = kmalloc_obj(*ptr, gfp);
ptr = kzalloc_obj(*ptr, gfp);
ptr = kmalloc_objs(*ptr, count, gfp);
ptr = kzalloc_objs(*ptr, count, gfp);

Beyond the other benefits outlined below, the primary ergonomic benefit
is the elimination of needing "sizeof" nor the type name, and the
enforcement of assignment types (they do not return "void *", but rather
a pointer to the type of the first argument). The type name _can_ be
used, though, in the case where an assignment is indirect (e.g. via
"return"). This additionally allows[1] variables to be declared via
__auto_type:

__auto_type ptr = kmalloc_obj(struct foo, gfp);

Internal introspection of the allocated type now becomes possible,
allowing for future alignment-aware choices to be made by the allocator
and future hardening work that can be type sensitive. For example,
adding __alignof(*ptr) as an argument to the internal allocators so that
appropriate/efficient alignment choices can be made, or being able to
correctly choose per-allocation offset randomization within a bucket
that does not break alignment requirements.

Link: https://lore.kernel.org/all/CAHk-=wiCOTW5UftUrAnvJkr6769D29tF7Of79gUjdQHS_TkF5A@mail.gmail.com/ [1]
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://patch.msgid.link/20251203233036.3212363-1-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>