scftorture: Forgive memory-allocation failure if KASAN

Kernels built with CONFIG_KASAN=y quarantine newly freed memory in order
to better detect use-after-free errors.  However, this can exhaust memory
more quickly in allocator-heavy tests, which can result in spurious
scftorture failure.  This commit therefore forgives memory-allocation
failure in kernels built with CONFIG_KASAN=y, but continues counting
the errors for use in detailed test-result analyses.

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