compressed pages

After loading a compressed-only page in buf_page_get_gen() we allocate a new
block for decompression. The problem is that the compressed page is neither
buffer-fixed nor I/O-fixed by the time we call buf_LRU_get_free_block(),
so it may end up being evicted and returned back as a new block.

buf_page_get_gen(): Temporarily buffer-fix the compressed-only block
while allocating memory for an uncompressed page frame.
This should prevent this form of the infinite loop, which is more likely
with a small innodb_buffer_pool_size.

rb#2511 approved by Jimmy Yang, Sunny Bains