MDEV-28247 : Disable background ibuf merge during Galera SST

This failure was caused by MDEV-25975, which removed the parameter
innodb_disallow_writes.

Added a check for wsrep_sst_disable_writes to the function
ibuf_merge_in_background().

storage/innobase/ibuf/ibuf0ibuf.cc

@@ -27,6 +27,13 @@ Created 7/19/1997 Heikki Tuuri
 #include "ibuf0ibuf.h"
 #include "sync0sync.h"
 #include "btr0sea.h"
+#ifdef WITH_WSREP
+extern uint32 wsrep_sst_disable_writes;
+# define wsrep_sst_disable_writes \
+  my_atomic_load32_explicit(&wsrep_sst_disable_writes, MY_MEMORY_ORDER_RELAXED)
+#else
+# define wsrep_sst_disable_writes false
+#endif
 
 using st_::span;
 
@@ -2653,6 +2660,10 @@ ibuf_merge_in_background(
 	}
 #endif /* UNIV_DEBUG || UNIV_IBUF_DEBUG */
 
+	if (wsrep_sst_disable_writes) {
+		return(0);
+	}
+
 	if (full) {
 		/* Caller has requested a full batch */
 		n_pages = PCT_IO(100);