zswap: dynamic pool creation

Add dynamic creation of pools.  Move the static crypto compression per-cpu
transforms into each pool.  Add a pointer to zswap_entry to the pool it's
in.

This is required by the following patch which enables changing the zswap
zpool and compressor params at runtime.

[akpm@linux-foundation.org: fix merge snafus]
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Acked-by: Seth Jennings <sjennings@variantweb.net>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>