MDEV-12353 preparation: Remove redundant writes

fsp_alloc_seg_inode_page(): Ever since
commit 3926673c
all newly allocated pages are zero-initialized.
Assert that this is the case for the FSEG_ID fields.
(Side note: before that fix, other parts of the pages
could contain nonzero garbage.)

btr_store_big_rec_extern_fields(): Remove the redundant initialization
of the most significant 32 bits of BTR_EXTERN_LEN. InnoDB never supported
BLOBs that are longer than 4GiB. In fact, dtuple_convert_big_rec()
would write emit an error message if a clustered index record tuple would
exceed 1,000,000,000 bytes in length.

storage/innobase/btr/btr0cur.cc

@@ -7870,8 +7870,8 @@ btr_store_big_rec_extern_fields(
 
 				extern_len -= store_len;
 
-				mlog_write_ulint(field_ref + BTR_EXTERN_LEN, 0,
-						 MLOG_4BYTES, &mtr);
+				ut_ad(!mach_read_from_4(BTR_EXTERN_LEN
+							+ field_ref));
 				mlog_write_ulint(field_ref
 						 + BTR_EXTERN_LEN + 4,
 						 big_rec_vec->fields[i].len

storage/innobase/fsp/fsp0fsp.cc

@@ -1539,9 +1539,7 @@ fsp_alloc_seg_inode_page(
 	fsp_header_t*	space_header,
 	mtr_t*		mtr)
 {
-	fseg_inode_t*	inode;
 	buf_block_t*	block;
-	page_t*		page;
 
 	ut_ad(page_offset(space_header) == FSP_HEADER_OFFSET);
 	ut_ad(page_get_space_id(page_align(space_header)) == space->id);
@@ -1556,24 +1554,20 @@ fsp_alloc_seg_inode_page(
 	buf_block_dbg_add_level(block, SYNC_FSP_PAGE);
 	ut_ad(rw_lock_get_sx_lock_count(&block->lock) == 1);
 
-	page = buf_block_get_frame(block);
-
-	mlog_write_ulint(page + FIL_PAGE_TYPE, FIL_PAGE_INODE,
+	mlog_write_ulint(block->frame + FIL_PAGE_TYPE, FIL_PAGE_INODE,
 			 MLOG_2BYTES, mtr);
 
-	const ulint physical_size = space->physical_size();
-
-	for (ulint i = 0; i < FSP_SEG_INODES_PER_PAGE(physical_size); i++) {
-
-		inode = fsp_seg_inode_page_get_nth_inode(
-			page, i, physical_size, mtr);
-
-		mlog_write_ull(inode + FSEG_ID, 0, mtr);
+#ifdef UNIV_DEBUG
+	const byte* inode = FSEG_ID + FSEG_ARR_OFFSET + block->frame;
+	for (ulint i = FSP_SEG_INODES_PER_PAGE(space->physical_size()); i--;
+	     inode += FSEG_INODE_SIZE) {
+		ut_ad(!mach_read_from_8(inode));
 	}
+#endif
 
 	flst_add_last(
 		space_header + FSP_SEG_INODES_FREE,
-		page + FSEG_INODE_PAGE_NODE, mtr);
+		block->frame + FSEG_INODE_PAGE_NODE, mtr);
 
 	return(true);
 }