mm, vma: use sheaves for vm_area_struct cache

mm, vma: use sheaves for vm_area_struct cache

Create the vm_area_struct cache with percpu sheaves of size 32 to
hopefully improve its performance. For CONFIG_PER_VMA_LOCK, change the
vma freeing from custom call_rcu() callback to kfree_rcu() which
will perform rcu_free sheaf batching. Since there may be additional
structures attached and they are freed only after the grace period,
create a __vma_area_rcu_free_dtor() to do that.

Note I have not investigated whether vma_numab_state_free() or
free_anon_vma_name() must really need to wait for the grace period. For
vma_lock_free() ideally we wouldn't free it at all when freeing the vma
to the sheaf (or even slab page), but that would require using also a
ctor for vmas to allocate the vma lock, and reintroducing dtor support
for deallocating the lock when freeing slab pages containing the vmas.

The plan is to move vma_lock into vma itself anyway, so if the rest can
be freed immediately, the whole destructor support won't be needed
anymore.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>