x86/vm86/32: Remove VM86_SCREEN_BITMAP support

The implementation was rather buggy.  It unconditionally marked PTEs
read-only, even for VM_SHARED mappings.  I'm not sure whether this is
actually a problem, but it certainly seems unwise.  More importantly, it
released the mmap lock before flushing the TLB, which could allow a racing
CoW operation to falsely believe that the underlying memory was not
writable.

I can't find any users at all of this mechanism, so just remove it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Stas Sergeev <stsp2@yandex.ru>
Link: https://lkml.kernel.org/r/f3086de0babcab36f69949b5780bde851f719bc8.1611078018.git.luto@kernel.org