mm: split SWP_FILE into SWP_ACTIVATED and SWP_FS

The SWP_FILE flag serves two purposes: to make swap_{read,write}page() go
through the filesystem, and to make swapoff() call ->swap_deactivate().
For Btrfs, we want the latter but not the former, so split this flag into
two.  This makes us always call ->swap_deactivate() if ->swap_activate()
succeeded, not just if it didn't add any swap extents itself.

This also resolves the issue of the very misleading name of SWP_FILE,
which is only used for swap files over NFS.

Link: http://lkml.kernel.org/r/6d63d8668c4287a4f6d203d65696e96f80abdfc7.1536704650.git.osandov@fb.com
Signed-off-by: Omar Sandoval <[email protected]>
Reviewed-by: Nikolay Borisov <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: David Sterba <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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