refscale: Do not disable interrupts for tests involving local_bh_enable()

refscale: Do not disable interrupts for tests involving local_bh_enable()

Some kernel configurations prohibit invoking local_bh_enable() while
interrupts are disabled.  However, refscale disables interrupts to reduce
OS noise during the tests, which results in splats.  This commit therefore
adds an ->enable_irqs flag to the ref_scale_ops structure, and refrains
from disabling interrupts when that flag is set.  This flag is set for
the "bh" and "incpercpubh" scale_type module-parameter values.

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