zsmalloc: replace per zpage lock with pool->migrate_lock

The zsmalloc has used a bit for spin_lock in zpage handle to keep
zpage object alive during several operations. However, it causes
the problem for PREEMPT_RT as well as introducing too complicated.

This patch replaces the bit spin_lock with pool->migrate_lock
rwlock. It could make the code simple as well as zsmalloc work
under PREEMPT_RT.

The drawback is the pool->migrate_lock is bigger granuarity than
per zpage lock so the contention would be higher than old when
both IO-related operations(i.e., zsmalloc, zsfree, zs_[map|unmap])
and compaction(page/zpage migration) are going in parallel(*,
the migrate_lock is rwlock and IO related functions are all read
side lock so there is no contention). However, the write-side
is fast enough(dominant overhead is just page copy) so it wouldn't
affect much. If the lock granurity becomes more problem later,
we could introduce table locks based on handle as a hash value.

Signed-off-by: Minchan Kim <minchan@kernel.org>