efi/x86: Don't map the entire kernel text RW for mixed mode

The mixed mode thunking routine requires a part of it to be
mapped 1:1, and for this reason, we currently map the entire
kernel .text read/write in the EFI page tables, which is bad.

In fact, the kernel_map_pages_in_pgd() invocation that installs
this mapping is entirely redundant, since all of DRAM is already
1:1 mapped read/write in the EFI page tables when we reach this
point, which means that .rodata is mapped read-write as well.

So let's remap both .text and .rodata read-only in the EFI
page tables.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200113172245.27925-6-ardb@kernel.org