MDEV-16469 SET GLOBAL innodb_change_buffering has no effect

When type of the settable global variable innodb_change_buffering was
changed from string to ENUM in MDEV-12218, the "shadow" variable ibuf_use
stopped being updated as a result of SET GLOBAL innodb_change_buffering.
Only on InnoDB startup, the parameter innodb_change_buffering would
take effect.

ibuf_use: Remove, and use the global variable innodb_change_buffering.

storage/innobase/handler/ha_innodb.cc

@@ -209,7 +209,6 @@ static char*	innobase_disable_monitor_counter;
 static char*	innobase_reset_monitor_counter;
 static char*	innobase_reset_all_monitor_counter;
 
-static ulong	innodb_change_buffering;
 static ulong	innodb_flush_method;
 
 /* This variable can be set in the server configure file, specifying
@@ -3859,7 +3858,6 @@ static int innodb_init_params()
 	}
 
 	DBUG_ASSERT(innodb_change_buffering <= IBUF_USE_ALL);
-	ibuf_use = ibuf_use_t(innodb_change_buffering);
 
 	/* Check that interdependent parameters have sane values. */
 	if (srv_max_buf_pool_modified_pct < srv_max_dirty_pages_pct_lwm) {

storage/innobase/ibuf/ibuf0ibuf.cc

@@ -185,7 +185,7 @@ it uses synchronous aio, it can access any pages, as long as it obeys the
 access order rules. */
 
 /** Operations that can currently be buffered. */
-ibuf_use_t	ibuf_use		= IBUF_USE_ALL;
+ulong	innodb_change_buffering;
 
 #if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
 /** Flag to control insert buffer debugging. */
@@ -3695,9 +3695,9 @@ ibuf_insert(
 	dberr_t		err;
 	ulint		entry_size;
 	ibool		no_counter;
-	/* Read the settable global variable ibuf_use only once in
+	/* Read the settable global variable only once in
 	this function, so that we will have a consistent view of it. */
-	ibuf_use_t	use		= ibuf_use;
+	ibuf_use_t	use		= ibuf_use_t(innodb_change_buffering);
 	DBUG_ENTER("ibuf_insert");
 
 	DBUG_PRINT("ibuf", ("op: %d, space: " UINT32PF ", page_no: " UINT32PF,

storage/innobase/include/ibuf0ibuf.h

@@ -61,7 +61,7 @@ enum ibuf_use_t {
 };
 
 /** Operations that can currently be buffered. */
-extern ibuf_use_t	ibuf_use;
+extern ulong		innodb_change_buffering;
 
 /** The insert buffer control structure */
 extern ibuf_t*		ibuf;

storage/innobase/include/ibuf0ibuf.ic

@@ -124,7 +124,7 @@ ibuf_should_try(
 						a secondary index when we
 						decide */
 {
-	return(ibuf_use != IBUF_USE_NONE
+	return(innodb_change_buffering
 	       && ibuf->max_size != 0
 	       && !dict_index_is_clust(index)
 	       && !dict_index_is_spatial(index)