Add kpreempt_disable/enable around CPU_SEQID uses

Add kpreempt_disable/enable around CPU_SEQID uses

In zfs/dmu_object and icp/core/kcf_sched, the CPU_SEQID macro
should be surrounded by `kpreempt_disable` and `kpreempt_enable`
calls to avoid a Linux kernel BUG warning.  These code paths use
the cpuid to minimize lock contention and is is safe to reschedule
the process to a different processor at any time.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Morgan Jones <me@numin.it>
Closes #6239