Linux compat 3.7.1, on_each_cpu()

Linux compat 3.7.1, on_each_cpu()

Some kernels require that we include the 'linux/irqflags.h'
header for the SPL_AC_3ARGS_ON_EACH_CPU check.  Otherwise,
the functions local_irq_enable()/local_irq_disable() will not
be defined and the prototype will be misdetected as the four
argument version.

This change actually include 'linux/interrupt.h' which in turn
includes 'linux/irqflags.h' to be as generic as possible.

Additionally, passing NULL as the function can result in a
gcc error because the on_each_cpu() macro executes it
unconditionally.  To make the test more robust we pass the
dummy function on_each_cpu_func().

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #204