btrfs-progs: do not allow setting seed flag on fs with dirty log

[BUG]
The following sequence operation can lead to a seed fs rejected by
kernel:

 # Generate a fs with dirty log
 mkfs.btrfs -f $file
 mount $dev $mnt
 xfs_io -f -c "pwrite 0 16k" -c fsync $mnt/file
 cp $file $file.backup
 umount $mnt
 mv $file.backup $file

 # now $file has dirty log, set seed flag on it
 btrfstune -S1 $file

 # mount will fail
 mount $file $mnt

The mount failure with the following dmesg:

[  980.363667] loop0: detected capacity change from 0 to 262144
[  980.371177] BTRFS info (device loop0): flagging fs with big metadata feature
[  980.372229] BTRFS info (device loop0): using free space tree
[  980.372639] BTRFS info (device loop0): has skinny extents
[  980.375075] BTRFS info (device loop0): start tree-log replay
[  980.375513] BTRFS warning (device loop0): log replay required on RO media
[  980.381652] BTRFS error (device loop0): open_ctree failed

[CAUSE]
Although btrfs will replay its dirty log even with RO mount, but kernel
will treat seed device as RO device, and dirty log can not be replayed
on RO device.

This rejection is already the better end, just imagine if we don't treat
seed device as RO, and replayed the dirty log.
The filesystem relying on the seed device will be completely screwed up.

[FIX]
Just add extra check on log tree in btrfstune to reject setting seed
flag on filesystems with dirty log.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

btrfstune.c

@@ -59,6 +59,10 @@ static int update_seeding_flag(struct btrfs_root *root, int set_flag)
 						device);
 			return 1;
 		}
+		if (btrfs_super_log_root(disk_super)) {
+			error("filesystem with dirty log detected, not setting seed flag");
+			return 1;
+		}
 		super_flags |= BTRFS_SUPER_FLAG_SEEDING;
 	} else {
 		if (!(super_flags & BTRFS_SUPER_FLAG_SEEDING)) {