erofs: use HIPRI by default if per-cpu kthreads are enabled

As Sandeep shown [1], high priority RT per-cpu kthreads are
typically helpful for Android scenarios to minimize the scheduling
latencies.

Switch EROFS_FS_PCPU_KTHREAD_HIPRI on by default if
EROFS_FS_PCPU_KTHREAD is on since it's the typical use cases for
EROFS_FS_PCPU_KTHREAD.

Also clean up unneeded sched_set_normal().

[1] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@mail.gmail.comReviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230522092141.124290-1-hsiangkao@linux.alibaba.com

fs/erofs/Kconfig

@@ -121,6 +121,7 @@ config EROFS_FS_PCPU_KTHREAD
 config EROFS_FS_PCPU_KTHREAD_HIPRI
 	bool "EROFS high priority per-CPU kthread workers"
 	depends on EROFS_FS_ZIP && EROFS_FS_PCPU_KTHREAD
+	default y
 	help
 	  This permits EROFS to configure per-CPU kthread workers to run
 	  at higher priority.

fs/erofs/zdata.c

@@ -369,8 +369,6 @@ static struct kthread_worker *erofs_init_percpu_worker(int cpu)
 		return worker;
 	if (IS_ENABLED(CONFIG_EROFS_FS_PCPU_KTHREAD_HIPRI))
 		sched_set_fifo_low(worker->task);
-	else
-		sched_set_normal(worker->task, 0);
 	return worker;
 }