page_create(): Create normal B-tree pages. Callers that create
R-tree pages will set FIL_PAGE_TYPE and reset the split
sequence number afterwards.

The creation of ROW_FORMAT=COMPRESSED pages is unaffected;
they will be logged as compressed page images.

page_create_low(): Take const buf_block_t* as a parameter.
Let the callers invoke buf_block_modify_clock_inc().