btrfs: fix crash in remount(thread_pool=) case
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 <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>
CC: Qu Wenruo <f41a7033a6382ce00d6b717e37d942dbe3c9a107@cn.fujitsu.com>
CC: Chris Mason <e2e8c5702d8ed5f565988a1cf5bb35c9ce199e5e@fb.com>
CC: Josef Bacik <631dfb3d07694fdcf26abc7aac2c6c2b641f8bde@fb.com>
CC: abe273e683b91ee2efa7b6f18db3864695dfa4c3@vger.kernel.org
Signed-off-by: Chris Mason <e2e8c5702d8ed5f565988a1cf5bb35c9ce199e5e@fb.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs