efi/x86: Mark kernel rodata non-executable for mixed mode

When remapping the kernel rodata section RO in the EFI pagetables, the
protection flags that were used for the text section are being reused,
but the rodata section should not be marked executable.

Cc: <stable@vger.kernel.org>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200717194526.3452089-1-nivedita@alum.mit.edu
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>