Fix truncation of filenames with suffixes for ISO9660

Include the suffix in the filename truncation so the combined length
remains within the filesystem limit, and truncate from the start of the
string to keep the suffix (the unique bit) to prevent kernel/initrd from
a particularly long filename from colliding.

app/services/foreman_bootdisk/iso_generator.rb

@@ -20,8 +20,7 @@ def self.generate_full_host(host, opts = {}, &block)
     files.map! do |bootfile_info|
       bootfile_info.map do |f|
         suffix = f[1].split('/').last
-        iso_suffix = iso9660_filename(suffix)
-        iso_f0 = iso9660_filename(f[0].to_s) + '_' + iso_suffix
+        iso_f0 = iso9660_filename(f[0].to_s + '_' + suffix)
         tmpl.gsub!(f[0].to_s + '-' + suffix, iso_f0)
         ForemanBootdisk.logger.debug("Boot file #{iso_f0}, source #{f[1]}")
         [iso_f0, f[1]]
@@ -134,7 +133,7 @@ def self.fetch(path, uri)
   # isolinux supports up to ISO 9660 level 2 filenames
   def self.iso9660_filename(name)
     dir  = File.dirname(name)
-    file = File.basename(name).upcase.tr_s('^A-Z0-9_', '_')[0..30]
-    dir == '.' ? file : File.join(dir.upcase.tr_s('^A-Z0-9_', '_')[0..30], file)
+    file = File.basename(name).upcase.tr_s('^A-Z0-9_', '_').last(28)
+    dir == '.' ? file : File.join(dir.upcase.tr_s('^A-Z0-9_', '_').last(28), file)
   end
 end