btrfs: fix crash in remount(thread_pool=) case

Reproducer:
mount /dev/ubda /mnt
mount -oremount,thread_pool=42 /mnt

Gives a crash:
? btrfs_workqueue_set_max+0x0/0x70
btrfs_resize_thread_pool+0xe3/0xf0
? sync_filesystem+0x0/0xc0
? btrfs_resize_thread_pool+0x0/0xf0
btrfs_remount+0x1d2/0x570
? kern_path+0x0/0x80
do_remount_sb+0xd9/0x1c0
do_mount+0x26a/0xbf0
? kfree+0x0/0x1b0
SyS_mount+0xc4/0x110

It's a call
btrfs_workqueue_set_max(fs_info->scrub_wr_completion_workers, new_pool_size);
with
fs_info->scrub_wr_completion_workers = NULL;

as scrub wqs get created only on user's demand.

Patch skips not-created-yet workqueues.

Signed-off-by: Sergei Trofimovich <[e-mail]>
CC: Qu Wenruo <[e-mail]>
CC: Chris Mason <[e-mail]>
CC: Josef Bacik <[e-mail]>
CC: [e-mail]
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/async-thread.c

···
 
 void btrfs_workqueue_set_max(struct btrfs_workqueue *wq, int max)
 {
+	if (!wq)
+		return;
 	wq->normal->max_active = max;
 	if (wq->high)
 		wq->high->max_active = max;