branches/zip: Do not decompress blocks in buf_page_init_for_read(),

but in buf_page_get_gen().  Adjust comments.  This prevents
unnecessary decompression in read-ahead when the compressed block
exists in the buffer pool.

If the block does not exist in the buffer pool, read-ahead will still
allocate an uncompressed page and decompress the block.

Move buf_block_init_low() and buf_zip_decompress() earlier in the file,
because some compilers are unable to inline functions that are defined
after the invocation.

buf_page_get_gen(): Decompress the block if needed.

buf_page_init_for_read(): Do not decompress.

buf/buf0lru.c

@@ -992,7 +992,7 @@ buf_LRU_free_block(
 			bpage->zip.data = NULL;
 			page_zip_set_size(&bpage->zip, 0);
 
-			/* Prevent buf_page_init_for_read() from
+			/* Prevent buf_page_get_gen() from
 			decompressing the block while we release
 			buf_pool->mutex and block_mutex. */
 			b->buf_fix_count++;

include/buf0buf.h

@@ -256,7 +256,7 @@ buf_page_get_gen(
 Initializes a page to the buffer buf_pool. The page is usually not read
 from a file even if it cannot be found in the buffer buf_pool. This is one
 of the functions which perform to a block a state transition NOT_USED =>
-FILE_PAGE (the other is buf_page_init_for_read above). */
+FILE_PAGE (others are buf_page_init_for_read and buf_page_get_gen). */
 
 buf_block_t*
 buf_page_create(