pgsql: Fix LOAD_CRIT_INDEX() macro to take out AccessShareLock on the

Log Message:
-----------
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.

Modified Files:
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    pgsql/src/backend/utils/cache:
        relcache.c (r1.270 -> r1.271)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c?r1=1.270&r2=1.271)