slab: don't batch kvfree_rcu() with SLUB_TINY

slab: don't batch kvfree_rcu() with SLUB_TINY

kvfree_rcu() is batched for better performance except on TINY_RCU, which
is a simple implementation for small UP systems. Similarly SLUB_TINY is
an option intended for small systems, whether or not used together with
TINY_RCU. In case SLUB_TINY is used with !TINY_RCU, it makes arguably
sense to not do the batching and limit the memory footprint. It's also
suboptimal to have RCU-specific #ifdefs in slab code.

With that, add CONFIG_KVFREE_RCU_BATCHED to determine whether batching
kvfree_rcu() implementation is used. It is not set by a user prompt, but
enabled by default and disabled in case TINY_RCU or SLUB_TINY are
enabled.

Use the new config for #ifdef's in slab code and extend their scope to
cover all code used by the batched kvfree_rcu(). For example there's no
need to perform kvfree_rcu_init() if the batching is disabled.

Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>