genfstab: canonicalize root dir

Avoids problems with users passing weird inputs (such as including a
trailing slash).

ref: https://bugs.archlinux.org/task/45753

genfstab.in

@@ -119,7 +119,7 @@ done
 shift $(( OPTIND - 1 ))
 
 (( $# )) || die "No root directory specified"
-root=$1; shift
+root=$(realpath -mL "$1"); shift
 
 if ! mountpoint -q "$root"; then
   die "$root is not a mountpoint"