zswap: Defer zswap initialisation

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.

See: https://github.com/raspberrypi/linux/pull/3432



Signed-off-by: Phil Elwell <phil@raspberrypi.com>