pgsql: Need to release buffer pins before attempting to drop files

Log Message:
-----------
Need to release buffer pins before attempting to drop files during
backend exit. Per report from Bruce.

Tags:
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REL8_0_STABLE

Modified Files:
--------------
pgsql/src/backend/utils/init:
postinit.c (r1.139 -> r1.139.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c.diff?r1=1.139&r2=1.139.4.1)