Fix #6213 - Method to_linux_x86_elf fails to set set :template

:template by default is just the base name of the file, not the
fullname. Before we use it, we need to normalize it. Methods
in this class rely on set_template_default for normalization (
which can also handle a custom path), so we'll just use that too.

Fix #6213

lib/msf/util/exe.rb

@@ -992,6 +992,11 @@ require 'msf/core/exe/segment_appender'
     if default
       elf = to_exe_elf(framework, opts, "template_x86_linux.bin", code)
     else
+      # Use set_template_default to normalize the :template key. It will just end up doing
+      # opts[:template] = File.join(opts[:template_path], opts[:template])
+      # for us, check if the file exists.
+      set_template_default(opts)
+
       # If this isn't our normal template, we have to do some fancy
       # header patching to mark the .text section rwx before putting our
       # payload into the entry point.