mm/zswap: delay the initialization of zswap

mm/zswap: delay the initialization of zswap

Since some users may not use zswap, the zswap_pool is wasted.  Save memory
by delaying the initialization of zswap until enabled.

[[email protected]: fix some pattern problem suggested by Christoph]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Liu Shixin <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>