mm: move responsibility for setting SWP_FS_OPS to ->swap_activate

If a filesystem wishes to handle all swap IO itself (via ->direct_IO and
->readpage), rather than just providing devices addresses for
submit_bio(), SWP_FS_OPS must be set.

Currently the protocol for setting this it to have ->swap_activate return
zero.  In that case SWP_FS_OPS is set, and add_swap_extent() is called for
the entire file.

This is a little clumsy as different return values for ->swap_activate
have quite different meanings, and it makes it hard to search for which
filesystems require SWP_FS_OPS to be set.

So remove the special meaning of a zero return, and require the filesystem
to set SWP_FS_OPS if it so desires, and to always call add_swap_extent()
as required.

Currently only NFS and CIFS return zero for add_swap_extent().

Link: https://lkml.kernel.org/r/164859778123.29473.17908205846599043598.stgit@noble.brown
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: David Howells <dhowells@redhat.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>