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 <slyfox@gentoo.org>
CC: Qu Wenruo <quwenruo@cn.fujitsu.com>
CC: Chris Mason <clm@fb.com>
CC: Josef Bacik <jbacik@fb.com>
CC: linux-btrfs@vger.kernel.org
Signed-off-by: Chris Mason <clm@fb.com>