buf_block_hash_get(): New function, similar to buf_page_hash_get().

buf_page_get_block(): Remove the const qualifiers.  This is a low-level
function, and the operations on block->mutex are non-const.

buf_page_try_get_func(): Implement with lower-level predicates, somewhat
similar to buf_page_get_known_nowait().

lock_rec_print(): Remove the unused variable zip_size and the
call to fil_space_get_zip_size().  Adapt to buf_page_try_get() returning
a const pointer.