lib/test_csd_lock: Add a module to stall a CPU on a CSD lock

Add test_csd_lock, a module that keeps one CPU from answering an IPI for
as long as its stall_ms parameter says, so that the CSD-lock debug code
has a stall to report.  With in_handler=1 the CPU stalls inside a CSD
handler instead, which is the case where the IPI is not re-sent.

The module needs CONFIG_CSD_LOCK_WAIT_DEBUG and csdlock_debug=1.  Loading
it runs one stall and then fails the load with -EAGAIN, the way
test_lockup does, so that nothing is left loaded afterwards.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>