zsmalloc: remove zspage isolation for migration

zspage isolation for migration introduced additional exceptions to be
dealt with since the zspage was isolated from class list.  The reason
why I isolated zspage from class list was to prevent race between
obj_malloc and page migration via allocating zpage from the zspage
further.  However, it couldn't prevent object freeing from zspage so it
needed corner case handling.

This patch removes the whole mess.  Now, we are fine since class->lock
and zspage->lock can prevent the race.

Link: https://lkml.kernel.org/r/20211115185909.3949505-7-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>