x86/gnttab: do not implement GNTTABOP_cache_flush

The current underlying implementation of GNTTABOP_cache_flush on x86 won't
work as expected.  The provided {clean,invalidate}_dcache_va_range()
helpers only do a local pCPU cache flush, so the cache of previous pCPUs
where the vCPU might have run are not flushed.

However instead of attempting to fix this, make the GNTTABOP_cache_flush
operation only available to ARM.  There are no known users on x86, the
implementation is broken, and other architectures don't have grant-table
support yet.

With that operation not implemented on x86, the related
{clean,invalidate}_dcache_va_range() helpers can also be removed.

Fixes: f62dc81c2d ("x86: introduce more cache maintenance operations")
Fixes: 18e8d22fe7 ("introduce GNTTABOP_cache_flush")
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-By: Oleksii Kurochko<oleksii.kurochko@gmail.com>
Acked-by: Julien Grall <jgrall@amazon.com>

CHANGELOG.md

@@ -28,6 +28,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
     - Ability to enable stack protector
 
 ### Removed
+ - On x86:
+   - GNTTABOP_cache_flush: it's unused on x86 and the implementation is
+     broken.
 
 ## [4.20.0](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.20.0) - 2025-03-05