mm/slub: unify all sl[au]b parameters with "slab_$param"

Since the SLAB allocator has been removed, so we can clean up the
sl[au]b_$params. With only one slab allocator left, it's better to use the
generic "slab" term instead of "slub" which is an implementation detail,
which is pointed out by Vlastimil Babka. For more information please see
[1]. Hence, we are going to use "slab_$param" as the primary prefix.

This patch is changing the following slab parameters
- slub_max_order
- slub_min_order
- slub_min_objects
- slub_debug
to
- slab_max_order
- slab_min_order
- slab_min_objects
- slab_debug
as the primary slab parameters for all references of them in docs and
comments. But this patch won't change variables and functions inside
slub as we will have wider slub/slab change.

Meanwhile, "slub_$params" can also be passed by command line, which is
to keep backward compatibility. Also mark all "slub_$params" as legacy.

Remove the separate descriptions for slub_[no]merge, append legacy tip
for them at the end of descriptions of slab_[no]merge.

[1] https://lore.kernel.org/linux-mm/7512b350-4317-21a0-fab3-4101bc4d8f7a@suse.cz/



Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>