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.

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