idr test suite: Create anchor before launching throbber

stable inclusion
from stable-5.10.31
commit b1f6c6f39bd6a47edcedf2532491ba26b212ecb3
bugzilla: 51792

--------------------------------

[ Upstream commit 094ffbd1 ]

The throbber could race with creation of the anchor entry and cause the
IDR to have zero entries in it, which would cause the test to fail.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by:   Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>