smb3: reenable swapfiles over SMB3 mounts

With the changes to folios/netfs it is now easier to reenable
swapfile support over SMB3 which fixes various xfstests

Reviewed-by: David Howells <dhowells@redhat.com>
Suggested-by: David Howells <dhowells@redhat.com>
Fixes: e1209d3a

 ("mm: introduce ->swap_rw and use it for reads from SWP_FS_OPS swap-space")
Signed-off-by: Steve French <stfrench@microsoft.com>