Avoid dev/inode check on btrfs with --sanity-level=3

Avoid dev/inode check on btrfs with --sanity-level=3

With --sanity-level=3 or higher the aspacemgr sanity checks the
device/inode numbers from /proc/self/maps to the file stat
results. These don't match on btrfs. So detect when a file is on a
btrfs volume and ignore the check in that case.

https://bugs.kde.org/show_bug.cgi?id=317127
(cherry picked from commit 3b06d458ffc5cc8de8d701926e5d86979185fa04)