xen/arm: mm: Don't open-code Xen PT update in {set, clear}_fixmap()

xen/arm: mm: Don't open-code Xen PT update in {set, clear}_fixmap()

{set, clear}_fixmap() are currently open-coding update to the Xen
page-tables. This can be avoided by using the generic helpers
map_pages_to_xen() and destroy_xen_mappings().

Both function are not meant to fail for fixmap, hence the BUG_ON()
checking the return.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>