zsmalloc: rename pool lock

zsmalloc: rename pool lock

The old name comes from the times when the pool did not have compaction
(defragmentation).  Rename it to ->lock because these days it synchronizes
not only migration.

Link: https://lkml.kernel.org/r/20250221222958.2225035-11-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>