pgsql: Tweak BgBufferSync() so that a persistent write error on a dirty

Log Message:
-----------
Tweak BgBufferSync() so that a persistent write error on a dirty buffer
doesn't block the bgwriter from making progress writing out other buffers.
This was a hard problem in the context of the ARC/2Q design, but it's
trivial in the context of clock sweep ... just advance the sweep counter
before we try to write not after.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.17&r2=1.18)
    pgsql/src/backend/storage/buffer:
        bufmgr.c (r1.189 -> r1.190)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.189&r2=1.190)