This function had a blacklist of dump object types that it believed
needed exclusive lock ... but we hadn't maintained that, so that it
was missing ROW SECURITY, POLICY, and INDEX ATTACH items, all of
which need (or should be treated as needing) exclusive lock.

Since the same oversight seems likely in future, let's reverse the
sense of the test so that the code has a whitelist of safe object
types; better to wrongly assume a command can't be run in parallel
than the opposite.  Currently the only POST_DATA object type that's
safe is CREATE INDEX ... and that list hasn't changed in a long time.

Back-patch to 9.5 where RLS came in.

Discussion: https://postgr.es/m/11450.1535483506@sss.pgh.pa.us