Avoid calling smp_processor_id in spl_magazine_age

The spl_magazine_age function had the implied assumption that it will
remain on its current cpu through its execution. In order to support
preempt enabled kernels, this assumption had to be removed.

The spl_kmem_magazine structure now holds the cpu id of the cpu it is
local to. This allows spl_magazine_age to use this field when scheduling
work to be done by the magazine's local cpu.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #98