btrfs: add regression test for remount with thread_pool resized

btrfs: add regression test for remount with thread_pool resized

Regression test for a btrfs issue of resizing 'thread_pool' when
remount the fs.

This regression was introduced by the following linux kernel commit:
    btrfs: Added btrfs_workqueue_struct implemented ordered
    execution based on kernel workqueue
    08a9ff3264181986d1d692a4e6fce3669700c9f8
And it was fixed by the following linux kernel commit:
    btrfs: fix crash in remount(thread_pool=) case
    800ee2247f483b6d05ed47ef3bbc90b56451746c

Signed-off-by: Xing Gu <gux.fnst@cn.fujitsu.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>