is_btrfs() returns incorrect value for btrfs filesystems directly on LUKS volumes

[sten0]

Affects master branch an[d] current stable release 0.5.

1. On a system where a btrfs filesystem exists directly on a LUKS volume.
2. Where this fs is mounted using /dev/mapper/foo in /etc/fstab.
3. btrfsmaintenance-functions:L21 returns '/dev/mapper/foo' rather than a mount point.
4. Which means 'stat -f --format=%T' in is_btrfs() returns tmpfs.
5. This means 'BTRFS_SCRUB_MOUNTPOINTS="auto"' in the config file skips all filesystems that match conditions one and two.
6. The user expectation for "auto" to work properly is not met.
7. Unfortunately, the is_btrfs check at btrfs-scrub.sh:39 means 'BTRFS_SCRUB_MOUNTPOINTS="/mount/point/for/LUKS_btrfs_fs"' will also fail, and thus cannot be used as a workaround. Additionally the expectation that an explicitly specified mount point will be scrubbed is not met.

Please fix this :)