pgsql: Fix bgwriter's failure to release buffer pins and open files

Log Message:
-----------
Fix bgwriter's failure to release buffer pins and open files after an
error.  This probably explains bug #2099 and could also account for
mysterious VACUUM hangups.

Tags:
----
REL8_1_STABLE

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.21 -> r1.21.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.21&r2=1.21.2.1)
    pgsql/src/backend/utils/resowner:
        resowner.c (r1.14.2.1 -> r1.14.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/resowner/resowner.c.diff?r1=1.14.2.1&r2=1.14.2.2)