MDEV-32983 cosmetic improvement on path separator near ib_buffer_pool

A mix of path separators looks odd.

  InnoDB: Loading buffer pool(s) from C:\xampp\mysql\data/ib_buffer_pool

This was changed in cf552f58

Both forward slashes and backward slashes work on Windows. We do not
use \\?\ names.

So we improve the consistent look of it so it doesn't look like a bug.

Normalize, in this case, the path separator to \ for making the filename.

Reported thanks to Github user @celestinoxp.

Closes: https://github.com/ApacheFriends/xampp-build/issues/33
Reviewed by: Marko Mäkelä and Vladislav Vaintroub

storage/innobase/buf/buf0dump.cc

@@ -180,7 +180,7 @@ static void buf_dump_generate_path(char *path, size_t path_size)
 	char	buf[FN_REFLEN];
 
 	mysql_mutex_lock(&LOCK_global_system_variables);
-	snprintf(buf, sizeof buf, "%s/%s", get_buf_dump_dir(),
+	snprintf(buf, sizeof buf, "%s" FN_ROOTDIR "%s", get_buf_dump_dir(),
 		 srv_buf_dump_filename);
 	mysql_mutex_unlock(&LOCK_global_system_variables);
 
@@ -214,7 +214,7 @@ static void buf_dump_generate_path(char *path, size_t path_size)
 			format = "%s%s";
 			break;
 		default:
-			format = "%s/%s";
+			format = "%s" FN_ROOTDIR "%s";
 		}
 
 		snprintf(path, path_size, format,