Btrfs: autodetect SSD devices

During mount, btrfs will check the queue nonrot flag
for all the devices found in the FS.  If they are all
non-rotating, SSD mode is enabled by default.

If the FS was mounted with -o nossd, the non-rotating
flag is ignored.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 7f5b288..3427db2 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -184,6 +184,7 @@
 		case Opt_nossd:
 			printk(KERN_INFO "btrfs: not using ssd allocation "
 			       "scheme\n");
+			btrfs_set_opt(info->mount_opt, NOSSD);
 			btrfs_clear_opt(info->mount_opt, SSD);
 			btrfs_clear_opt(info->mount_opt, SSD_SPREAD);
 			break;
@@ -438,6 +439,8 @@
 		seq_printf(seq, ",thread_pool=%d", info->thread_pool_size);
 	if (btrfs_test_opt(root, COMPRESS))
 		seq_puts(seq, ",compress");
+	if (btrfs_test_opt(root, NOSSD))
+		seq_puts(seq, ",nossd");
 	if (btrfs_test_opt(root, SSD_SPREAD))
 		seq_puts(seq, ",ssd_spread");
 	else if (btrfs_test_opt(root, SSD))