xen/arm: mm: Access a PT entry before the table is unmapped

xen/arm: mm: Access a PT entry before the table is unmapped

xen_pt_next_level() will retrieve the MFN from the entry right after the
page-table has been unmapped.

After calling xen_unmap_table(), there is no guarantee the mapping will
still be valid. Depending on the implementation, this may result to a
data abort in Xen.

Re-order the code to retrieve the MFN before the table is unmapped.

Fixes: 53abb9a1dcd9 ("xen/arm: mm: Rework Xen page-tables walk during update")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Paul Durrant <paul@xen.org>