mm, slob: rename CONFIG_SLOB to CONFIG_SLOB_DEPRECATED

As explained in [1], we would like to remove SLOB if possible.

- There are no known users that need its somewhat lower memory footprint
  so much that they cannot handle SLUB (after some modifications by the
  previous patches) instead.

- It is an extra maintenance burden, and a number of features are
  incompatible with it.

- It blocks the API improvement of allowing kfree() on objects allocated
  via kmem_cache_alloc().

As the first step, rename the CONFIG_SLOB option in the slab allocator
configuration choice to CONFIG_SLOB_DEPRECATED. Add CONFIG_SLOB
depending on CONFIG_SLOB_DEPRECATED as an internal option to avoid code
churn. This will cause existing .config files and defconfigs with
CONFIG_SLOB=y to silently switch to the default (and recommended
replacement) SLUB, while still allowing SLOB to be configured by anyone
that notices and needs it. But those should contact the slab maintainers
and [email protected] as explained in the updated help. With no valid
objections, the plan is to update the existing defconfigs to SLUB and
remove SLOB in a few cycles.

To make SLUB more suitable replacement for SLOB, a CONFIG_SLUB_TINY
option was introduced to limit SLUB's memory overhead.
There is a number of defconfigs specifying CONFIG_SLOB=y. As part of
this patch, update them to select CONFIG_SLUB and CONFIG_SLUB_TINY.

[1] https://lore.kernel.org/all/[email protected]/

Cc: Russell King <[email protected]>
Cc: Aaro Koskinen <[email protected]>
Cc: Janusz Krzysztofik <[email protected]>
Cc: Tony Lindgren <[email protected]>
Cc: Jonas Bonn <[email protected]>
Cc: Stefan Kristiansson <[email protected]>
Cc: Stafford Horne <[email protected]>
Cc: Yoshinori Sato <[email protected]>
Cc: Rich Felker <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Conor Dooley <[email protected]>
Cc: Damien Le Moal <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Aaro Koskinen <[email protected]> # OMAP1
Reviewed-by: Damien Le Moal <[email protected]> # riscv k210
Acked-by: Arnd Bergmann <[email protected]> # arm
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Mike Rapoport <[email protected]>
Reviewed-by: Christoph Lameter <[email protected]>

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