smb: remove SLAB_MEM_SPREAD flag usage

The SLAB_MEM_SPREAD flag is already a no-op as of 6.8-rc1, remove
its usage so we can delete it from slab. No functional change.

Link: https://lore.kernel.org/all/20240223-slab-cleanup-flags-v2-0-02f1753e8303@suse.cz/

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Steve French <stfrench@microsoft.com>