Fix parallel amvacuumcleanup safety bug.

Fix parallel amvacuumcleanup safety bug.

Commit b4af70cb inverted the return value of the function
parallel_processing_is_safe(), but missed the amvacuumcleanup test.
Index AMs that don't support parallel cleanup at all were affected.

The practical consequences of this bug were not very serious.  Hash
indexes are affected, but since they just return the number of blocks
during hashvacuumcleanup anyway, it can't have had much impact.

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAD21AoA-Em+aeVPmBbL_s1V-ghsJQSxYL-i3JP8nTfPiD1wjKw@mail.gmail.com
Backpatch: 14-, where commit b4af70cb appears.