arm64: mm: Change page table pointer name in p[md]_set_huge()

This is preparation for the following backported fixes.  It was done
upstream as part of commit 20a004e7 "arm64: mm: Use
READ_ONCE/WRITE_ONCE when accessing page tables", the rest of which
does not seem suitable for stable.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>