dtrace: Deduplicate dtrace_sync() and dtrace_xcall() implementations

dtrace_xcall() is just a thin wrapper around smp_rendezvous_cpus().
There's no need for six identical implementations to live in MD layers.

No functional change intended.

MFC after: 2 weeks

(cherry picked from commit bf115203bb8a61bd03ba23931ff0b5bf931b7d1b)