dict_check_sys_tables(): Do not rely on buf_page_optimistic_get()

We are supposed to commit and restart the mini-transaction
between records. There is no point to store and restore the
persistent cursor position otherwise.

If buf_page_optimistic_get() is patched to always fail, the
debug build would fail to start up due to trying to re-acquire
an already S-latched block.

This bug (which should not have visible impact to users, because
the code is only executed during startup, while no other threads
are accessing B-trees or causing pages to be evicted from the
buffer pool) was caught as part of a debugging effort for
something else.

The debugging approach was: Make buf_page_optimistic_get()
always return FALSE, and add ut_a(block->lock.lock_word == X_LOCK_DECR)
to both buf_LRU_get_free_only() and buf_LRU_block_free_non_file_page().
This would catch misuse of the buffer pool. If it were not for
buf_page_optimistic_get(), no buf_block_t::lock of any BUF_BLOCK_NOT_USED
block would ever be acquired.

storage/innobase/dict/dict0load.cc

@@ -1388,6 +1388,7 @@ static ulint dict_check_sys_tables()
 
 	for (rec = dict_startscan_system(&pcur, &mtr, SYS_TABLES);
 	     rec != NULL;
+	     mtr.commit(), mtr.start(),
 	     rec = dict_getnext_system(&pcur, &mtr)) {
 		const byte*	field;
 		ulint		len;