xen/arm: Pair call to set_fixmap with call to clear_fixmap in copy_from_paddr

xen/arm: Pair call to set_fixmap with call to clear_fixmap in copy_from_paddr

At the moment, set_fixmap may replace a valid entry without following
the break-before-make sequence. This may result to TLB conflict abort.

Rather than dealing with Break-Before-Make in set_fixmap, each call to
set_fixmap in copy_from_paddr is paired with a call to clear_fixmap.

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