MDEV-28111 Redo log writes are being buffered on Linux for no good reason

In commit 685d958e (MDEV-14425)
we ended up not enabling O_DIRECT writes on the redo log
by default, because back then, it was slightly slower on
some systems.

With commit a635c406 (MDEV-27774)
the situation changed. A new test on a NVMe device shows 9%
improvement in throughput and over 15% reduction of latency
when O_DIRECT writes are enabled.

With this change, all the following settings will use O_DIRECT
on InnoDB data and log files:

innodb_flush_method=O_DIRECT
innodb_flush_method=O_DIRECT_NO_FSYNC
innodb_flush_method=O_DSYNC

Before MDEV-14425, log writes were always buffered on Linux.
Between MDEV-14425 and this change, unbuffered log writes
were only enabled for innodb_flush_method=O_DSYNC.

storage/innobase/os/os0file.cc

@@ -1290,13 +1290,13 @@ os_file_create_func(
 
 #if (defined(UNIV_SOLARIS) && defined(DIRECTIO_ON)) || defined O_DIRECT
 	if (type == OS_DATA_FILE) {
+# ifdef __linux__
+use_o_direct:
+# endif
 		switch (srv_file_flush_method) {
 		case SRV_O_DSYNC:
 		case SRV_O_DIRECT:
 		case SRV_O_DIRECT_NO_FSYNC:
-# ifdef __linux__
-use_o_direct:
-# endif
 			os_file_set_nocache(file, name, mode_str);
 			break;
 		default:
@@ -1349,9 +1349,7 @@ os_file_create_func(
 				goto skip_o_direct;
 			}
 			log_sys.set_block_size(uint32_t(s));
-			if (srv_file_flush_method == SRV_O_DSYNC) {
-				goto use_o_direct;
-			}
+			goto use_o_direct;
 		} else {
 skip_o_direct:
 			log_sys.set_block_size(0);