Turn kmem_cache_alloc() into a wrapper around kmem_cache_alloc_lru().

Fixes: 9bbdc0f3 ("xarray: use kmem_cache_alloc_lru to allocate xa_node")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: Li Wang <liwang@redhat.com>