pgsql-server: Fix incorrect ordering of smgr cleanup relative to buffer

Log Message:
-----------
Fix incorrect ordering of smgr cleanup relative to buffer pin cleanup
during transaction abort.  Add a regression test case to catch related
mistakes in future.  Alvaro Herrera and Tom Lane.

Modified Files:
--------------
    pgsql-server/src/backend/access/transam:
        xact.c (r1.185 -> r1.186)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.185&r2=1.186)
    pgsql-server/src/backend/storage/smgr:
        smgr.c (r1.81 -> r1.82)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/smgr/smgr.c.diff?r1=1.81&r2=1.82)
    pgsql-server/src/test/regress/expected:
        transactions.out (r1.8 -> r1.9)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/test/regress/expected/transactions.out.diff?r1=1.8&r2=1.9)
    pgsql-server/src/test/regress/sql:
        transactions.sql (r1.8 -> r1.9)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/test/regress/sql/transactions.sql.diff?r1=1.8&r2=1.9)