Add support for detecting when btrfs is for root file system

Proposed by forum member @Aragorn in https://forum.manjaro.org/t/patch-add-btrfs-support-to-manjaro-chroot-a/178765

The change introduce a local variable $fstab in function lib/util-mount/parse_fstab() which is preset to /etc/fstab.

A conditional check is introduced and if this evaluates true the file system is btrfs and the fstab variable is changed to /@/etc/fstab

Although I have searched the source for other references - please verify this is not affecting any part I could not identify.