blk-wbt: don't throttle swap writes in direct reclaim

Now we avoid throttling swap writes by determining whether the current
process is kswapd (aka current_is_kswapd()), but swap writes can come
from either kswapd or direct reclaim, so the swap writes from direct
reclaim will still be throttled.

When a process holds a lock to allocate a free page, and enters direct
reclaim because there is no free memory, then it might trigger a hung
due to the wbt throttling that causes other processes to fail to get
the lock.

Both kswapd and direct reclaim set the REQ_SWAP flag, so use REQ_SWAP
instead of current_is_kswapd() to avoid throttling swap writes. Also
renamed WBT_KSWAPD to WBT_SWAP and WBT_RWQ_KSWAPD to WBT_RWQ_SWAP.

Signed-off-by: Baokun Li <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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