radix tree test suite: Register the main thread with the RCU library

radix tree test suite: Register the main thread with the RCU library

[ Upstream commit 1bb4bd266cf39fd2fa711f2d265c558b92df1119 ]

Several test runners register individual worker threads with the
RCU library, but neglect to register the main thread, which can lead
to objects being freed while the main thread is in what appears to be
an RCU critical section.

Reported-by: Chris von Recklinghausen <crecklin@redhat.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>