pgsql: Separate the functions of relcache entry flush and smgr cache

Log Message:
-----------
Separate the functions of relcache entry flush and smgr cache entry flush
so that we can get the size of a shared inval message back down to what it
was in 7.4 (and simplify the logic too).  Phase 2 of fixing the
'SMgrRelation hashtable corrupted' problem.

Modified Files:
--------------
    pgsql/src/backend/utils/cache:
        inval.c (r1.69 -> r1.70)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/inval.c.diff?r1=1.69&r2=1.70)
    pgsql/src/include/storage:
        sinval.h (r1.39 -> r1.40)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/sinval.h.diff?r1=1.39&r2=1.40)