install-grub.pl: support bindmounts and btrfs subvolumes

Specifically this allows bind-mounting your `/boot` directory with
NixOS impermanence.

nixos/modules/system/boot/loader/grub/install-grub.pl

@@ -110,19 +110,20 @@ sub runCommand {
 # Discover information about the location of the bootPath
 struct(Fs => {
     device => '$',
+    root => '$',
     type => '$',
     mount => '$',
 });
 
 # Figure out what filesystem is used for the directory with init/initrd/kernel files
 sub GetFs {
     my ($dir) = @_;
-    my ($status, @pathInfo) = runCommand("@utillinux@/bin/findmnt", "-n", "-v", "-o", "SOURCE,FSTYPE,TARGET", "-T", @{[$dir]});
+    my ($status, @pathInfo) = runCommand("@utillinux@/bin/findmnt", "-n", "-v", "-o", "SOURCE,FSROOT,FSTYPE,TARGET", "-T", @{[$dir]});
     if ($status != 0 || @pathInfo != 1) {
         die "Failed to get file system (returned $status) for @{[$dir]}";
     }
     my @fields = split /\s+/, $pathInfo[0];
-    return Fs->new(device => $fields[0], type => $fields[1], mount => $fields[2]);
+    return Fs->new(device => $fields[0], root => $fields[1], type => $fields[2], mount => $fields[3]);
 }
 struct (Grub => {
     path => '$',
@@ -132,10 +133,7 @@ sub GetFs {
 sub GrubFs {
     my ($dir) = @_;
     my $fs = GetFs($dir);
-    my $path = substr($dir, length($fs->mount));
-    if (substr($path, 0, 1) ne "/") {
-        $path = "/$path";
-    }
+    my $path = $fs->root . substr($dir, length($fs->mount));
     my $search = "";
 
     # ZFS is completely separate logic as zpools are always identified by a label