mtr_commit(): Do not acquire the log mutex if no log records are to be written.

ibuf_set_free_bits_func(): Do not clear mtr.modifications, because that would
prevent the dirty insert buffer bitmap block from being moved to the flush
list.  Instead, assume that mtr_commit() will not acquire log_sys->mutex
because redo logging is disabled.

buf_page_init_for_read(): I/O-fix the block before buf_buddy_alloc(), because
the newly allocated block could otherwise be reused by the compressed page.