RFC: Test of zswap deferred initialisation (take 2)

[pelwell]

This PR is the result of some experimentation to try and defer the cost
of enabling the ZSWAP configuration option until zswap is actually
enabled. There is no guarantee it will get merged, but some feedback
that zswap still works and that the overhead is acceptable would make
it more likely.

It differs from #3600 in that more initialisation is done at the init stage
(allowing more memory to be released after boot), and it is also based on
the 5.4 kernel rather than 4.19.

See also the discussion at #3432.

[ From the main commit message ]

Enabling zswap support in the kernel configuration costs about 1.5MB
of RAM, even when zswap is not enabled at runtime. This cost can be
reduced significantly by deferring initialisation (including pool
creation) until the "enabled" parameter is set to true. There is a
small cost to this in that some initialisation code has to remain in
memory after the init phase, just in case they are needed later,
but the total size increase is negligible.