scftorture: Pause testing after memory-allocation failure

The scftorture test can quickly execute a large number of calls to no-wait
smp_call_function(), each of which holds a block of memory until the
corresponding handler is invoked.  Especially when the longwait module
parameter is specified, this can chew up an arbitrarily large amount
of memory.  This commit therefore blocks after each memory-allocation
failure, with the duration a function of longwait.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>