buf_page_struct: Note that space and offset are also protected by
buf_pool_mutex.  They are only assigned to by
buf_block_set_file_page().  Thus, it suffices for buf_flush_batch() to
hold just buf_pool_mutex when checking these fields.

buf_flush_try_page(): Rename "locked" to "is_s_latched", per Heikki's request.

buf_flush_batch(): Move the common statement mutex_exit(block_mutex)
from all if-else if-else branches before the if block.  Remove the
redundant test (buf_pool->init_flush[flush_type] == FALSE) that was
apparently copied from buf_flush_write_complete().

buf_flush_write_block_low(): Note why it is safe not to hold buf_pool_mutex
or block_mutex.  Enumerate the assumptions in debug assertions.