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.

Modified Files:
--------------
pgsql/src/backend/utils/init:
postinit.c (r1.141 -> r1.142)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c.diff?r1=1.141&r2=1.142)