buf_flush_try_page(): Rename to buf_flush_page(), and change the
return type to void.  Replace the parameters space, offset with bpage,
and remove the second page hash lookup.  Note and assert that both
buf_pool_mutex and block_mutex must now be held upon entering the
function.  They will still be released by this function.

buf_flush_try_neighbors(): Replace buf_flush_try_page() with
buf_flush_page().  Make the logic easier to follow by not negating the
precondition of buf_flush_page().

rb://73 approved by Sunny Bains.  This is related to Issue #157.