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

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.