MDEV-23399 fixup for MDEV-10814

buf_madvise_do_dump(): Fix a mutex release that was broken in
commit 7cffb5f6.
This function is not covered by any tests. Its only purpose is
to be called from a debugger so that buffers that would normally
be excluded from core dumps ever since
commit b600f307
can be included.

storage/innobase/buf/buf0buf.cc

@@ -1021,7 +1021,7 @@ buf_madvise_do_dump()
 		ret+= madvise(chunk->mem, chunk->mem_size(), MADV_DODUMP);
 	}
 
-	mysql_mutex_lock(&buf_pool.mutex);
+	mysql_mutex_unlock(&buf_pool.mutex);
 	return ret;
 }
 #endif