Fix invalid pointer access in trace_dbuf.h

In dnode_destroy, dn_objset is invalidated. However, it will later call
into dbuf_destroy, in which DTRACE_SET_STATE will try to access spa_name
via dn_objset causing illegal pointer access.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #15333