systemd-sleep: refuse to calculate swapfile offset on Btrfs

systemd-sleep: refuse to calculate swapfile offset on Btrfs

If hibernation is requested but /sys/power/resume and
/sys/power/resume_offset are not configured correctly, systemd-sleep
attempts to calculate swapfile offset using fstat and fiemap.
Btrfs returns virtual device number for stat and a virtual offset
for fiemap which results in incorrect offset calculations. In the
case where offset would be calculated and the user is using Btrfs,
log a debug message and fail to write device and offset values.