Fix a couple of issues with the way we map and copy the vendor string:
- we map only 2 bytes, which usually works since you get at least a
  page, but if the vendor string happens to cross a page boundary,
  a crash will result
- only call early_memunmap() if early_memremap() succeeded, or we will
  call it with a NULL address which it doesn't like,
- while at it, switch to early_memremap_ro(), and array indexing rather
  than pointer dereferencing to read the CHAR16 characters.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Matthew Garrett <mjg59@google.com>
Cc: linux-efi@vger.kernel.org
Fixes: 5b83683f ("x86: EFI runtime service support")
Link: https://lkml.kernel.org/r/20200103113953.9571-5-ardb@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>