dracut.sh: don't call fsfreeze on subvol of root file system

dracut.sh: don't call fsfreeze on subvol of root file system

dracut.sh already doesn't call fsfreeze if the output file is on
the root file system. For btrfs, however, this is not sufficient.
Because fsfreeze is a superblock operation, and all btrfs subvolumes
share the same superblock, fsfreeze may freeze the entire system
if the subvolume on which the output file is written and / are
subvolumes of the same file system. Avoid this by comparing file
system UUIDs for btrfs.

Fixes: de576db3c225 ("call fsfreeze(8) on /boot to flush initramfs data & metadata to media")