efi_loader: fix memory allocation on sandbox

Commit 7b78d643 ("efi_loader: Reserve unaccessible memory") introduced
a comparison between RAM top and RAM start that was not known at the time
when the patch of commit 49759743 ("efi_loader: eliminate sandbox
addresses") was written.

The sandbox uses an address space that is only relevant in the sandbox
context. We have to map ram_top from the sandbox address space to the
physical address space before using it in the EFI subsystem.

Fixes: 49759743 ("efi_loader: eliminate sandbox addresses")
Fixes: 7b78d643 ("efi_loader: Reserve unaccessible memory")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>