create-diff-object: don't create __fentry__ dynrelas

This fixes the following issue for a patch which changes a module:

  kpatch_create_mcount_sections: 1968: bad first rela in .rela.text.e_show

The first rela is "bad" because the real first rela was converted to a
dynrela.

This is a temporary fix.  The more permanent fix should be to allow
lookups in vmlinux for patched modules so we don't create any
unnecessary dynrelas.