buf_LRU_get_free_block(): Document that the block is in the state
BUF_BLOCK_READY_FOR_USE.

buf_block_alloc(): Change the state of the block to BUF_BLOCK_MEMORY
and document it.

Replace those invocations of buf_block_alloc() with
buf_LRU_get_free_block() where the allocated block is used for buffer
pool input and output.  However, temporary copies of B-tree pages
during reorganization are not used for file I/O, and such blocks are
still allocated with buf_block_alloc().