_damo_fs: compare with s_subtype instead for mnt_devname

[onlyoneofme]

In case of sysfs its /proc/mounts entry looks different from debugfs:

sys /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0

It may have sys as mnt_devname (a.k.a dev_name), so searching for it with sysfs gives no result.

[sj-aws]

Looks good to me, thank you so much!