Dept. of second thoughts: fix loop in BgBufferSync so that the exit when

Dept. of second thoughts: fix loop in BgBufferSync so that the exit when
bgwriter_lru_maxpages is exceeded leaves the loop variables in the
expected state.  In the original coding, we'd fail to advance
next_to_clean, causing that buffer to be probably-uselessly rechecked next
time, and also have an off-by-one idea of the number of buffers scanned.