pgsql: Do all accesses to shared buffer headers through

Log Message:
-----------
Do all accesses to shared buffer headers through volatile-qualified
pointers, to ensure that compilers won't rearrange accesses to occur
while we're not holding the buffer header spinlock.  It's probably
not necessary to mark volatile in every single place in bufmgr.c,
but better safe than sorry.  Per trouble report from Kevin Grittner.

Modified Files:
--------------
    pgsql/contrib/pg_buffercache:
        pg_buffercache_pages.c (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_buffercache/pg_buffercache_pages.c.diff?r1=1.4&r2=1.5)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.195 -> r1.196)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.195&r2=1.196)
        freelist.c (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/freelist.c.diff?r1=1.52&r2=1.53)
    pgsql/src/include/storage:
        buf_internals.h (r1.79 -> r1.80)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/buf_internals.h.diff?r1=1.79&r2=1.80)