Allow Xen tmem shim to be built/loaded as a module. Xen self-ballooning
and frontswap-selfshrinking are now also "lazily" initialized when the
Xen tmem shim is loaded as a module, unless explicitly disabled by
module parameters.

Note runtime dependency disallows loading if cleancache/frontswap lazy
initialization patches are not present.

If built-in (not built as a module), the original mechanism of enabling
via a kernel boot parameter is retained, but this should be considered
deprecated.

Note that module unload is explicitly not yet supported.

[v1: Removed the [CLEANCACHE|FRONTSWAP]_HAS_LAZY_INIT ifdef]
[v2: Squashed the xen/tmem: Remove the subsys call patch in]
[akpm@linux-foundation.org: fix build (disable_frontswap_selfshrinking undeclared)]
Signed-off-by: Dan Magenheimer
Signed-off-by: Konrad Rzeszutek Wilk
Signed-off-by: Bob Liu
Cc: Wanpeng Li
Cc: Andor Daam
Cc: Florian Schmaus
Cc: Minchan Kim
Cc: Stefan Hengelein
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds