PR14682: fix _stp_pmap_agg error exit path

PR14682: fix _stp_pmap_agg error exit path

commit 82523f19 changed the error-exit path of _stp_pmap_agg, but was
confused by the multiple (three!) levels of nested loops in effect at
the point of failure.  While the prior "return;" skipped an overall
(newly needed) aggregate-unlock; the current "break;" skipped too
little.  Switch to a proper simple goto to almost but not quite
return;.