Fix CPU_SEQID use in preemptible context

Commit e022864 introduced a regression for kernels which are built
with CONFIG_DEBUG_PREEMPT.  The use of CPU_SEQID in a preemptible
context causes zio_nowait() to trigger the BUG.  Since CPU_SEQID
is simply being used as a random index the usage here is safe. To
resolve the issue preempt is disable while calling CPU_SEQID.