Fix LOAD_CRIT_INDEX() macro to take out AccessShareLock on the system index

Fix LOAD_CRIT_INDEX() macro to take out AccessShareLock on the system index
it is trying to build a relcache entry for.  This is an oversight in my 8.2
patch that tried to ensure we always took a lock on a relation before trying
to build its relcache entry.  The implication is that if someone committed a
reindex of a critical system index at about the same time that some other
backend were starting up without a valid pg_internal.init file, the second one
might PANIC due to not seeing any valid version of the index's pg_class row.
Improbable case, but definitely not impossible.