install-grub.pl: use findmnt to get the boot filesystem

The current implementation of GetFS uses heuristics to find the best
filesystem for the given boot path. Using findmnt is more robust.

Specifically, this fixes mounting tmpfs on / for [NixOS
impermanence](https://github.com/nix-community/impermanence).

Before this change, GetFs would incorrectly match /boot to the tmpfs
filesystem, resulting in a broken grub config.

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

@@ -113,56 +113,16 @@ sub runCommand {
     type => '$',
     mount => '$',
 });
-sub PathInMount {
-    my ($path, $mount) = @_;
-    my @splitMount = split /\//, $mount;
-    my @splitPath = split /\//, $path;
-    if ($#splitPath < $#splitMount) {
-        return 0;
-    }
-    for (my $i = 0; $i <= $#splitMount; $i++) {
-        if ($splitMount[$i] ne $splitPath[$i]) {
-            return 0;
-        }
-    }
-    return 1;
-}
 
 # Figure out what filesystem is used for the directory with init/initrd/kernel files
 sub GetFs {
     my ($dir) = @_;
-    my $bestFs = Fs->new(device => "", type => "", mount => "");
-    foreach my $fs (read_file("/proc/self/mountinfo")) {
-        chomp $fs;
-        my @fields = split / /, $fs;
-        my $mountPoint = $fields[4];
-        my @mountOptions = split /,/, $fields[5];
-
-        # Skip the optional fields.
-        my $n = 6; $n++ while $fields[$n] ne "-"; $n++;
-        my $fsType = $fields[$n];
-        my $device = $fields[$n + 1];
-        my @superOptions = split /,/, $fields[$n + 2];
-
-        # Skip the bind-mount on /nix/store.
-        next if $mountPoint eq "/nix/store" && (grep { $_ eq "rw" } @superOptions);
-        # Skip mount point generated by systemd-efi-boot-generator?
-        next if $fsType eq "autofs";
-
-        # Ensure this matches the intended directory
-        next unless PathInMount($dir, $mountPoint);
-
-        # Is it better than our current match?
-        if (length($mountPoint) > length($bestFs->mount)) {
-
-            # -d performs a stat, which can hang forever on network file systems,
-            # so we only make this call last, when it's likely that this is the mount point we need.
-            next unless -d $mountPoint;
-
-            $bestFs = Fs->new(device => $device, type => $fsType, mount => $mountPoint);
-        }
+    my ($status, @pathInfo) = runCommand("@utillinux@/bin/findmnt", "-n", "-v", "-o", "SOURCE,FSTYPE,TARGET", "-T", @{[$dir]});
+    if ($status != 0 || @pathInfo != 1) {
+        die "Failed to get file system (returned $status) for @{[$dir]}";
     }
-    return $bestFs;
+    my @fields = split /\s+/, $pathInfo[0];
+    return Fs->new(device => $fields[0], type => $fields[1], mount => $fields[2]);
 }
 struct (Grub => {
     path => '$',