autovacuum: Fix polarity of "wraparound" variable

Commit 0d831389 inadvertently reversed the meaning of the
wraparound variable.  This causes vacuums which are not required for
wraparound to wait for locks to be acquired, and what is worse, it
allows wraparound vacuums to skip locked pages.

Bug reported by Jeff Janes in
http://www.postgresql.org/message-id/CAMkU=1xmTEiaY=5oMHsSQo5vd9V1Ze4kNLL0qN2eH0P_GXOaYw@mail.gmail.com
Analysis and patch by Kyotaro HORIGUCHI