btr_free_root(): Relax a too strict debug assertion

When btr_create() invokes btr_free_root() after running out of space,
fseg_create() would have acquired an SX-latch on the root page, not
an X-latch. Relax the debug assertion in btr_free_root() accordingly.

(In this case, SX-latch and X-latch are equivalent. During the CREATE
operation there should be MDL_EXCLUSIVE and dict_operation_lock X-latch
preventing concurrent access to the index. Normally the purpose of the
SX-latch is to allow concurrent reads of the root page while certain
fields in the root page are updated in place.)

storage/innobase/btr/btr0btr.cc

@@ -1041,7 +1041,8 @@ btr_free_root(
 {
 	fseg_header_t*	header;
 
-	ut_ad(mtr_memo_contains_flagged(mtr, block, MTR_MEMO_PAGE_X_FIX));
+	ut_ad(mtr_memo_contains_flagged(mtr, block, MTR_MEMO_PAGE_X_FIX
+					| MTR_MEMO_PAGE_SX_FIX));
 	ut_ad(mtr->is_named_space(block->page.id.space()));
 
 	btr_search_drop_page_hash_index(block);