xen/arm: fix nr_pdxs calculation

xen/arm: fix nr_pdxs calculation

pfn_to_pdx expects an address, not a size, as a parameter. Specifically,
it expects the end address, then the masks calculations compensate for
any holes between start and end. Thus, we should pass the end address to
pfn_to_pdx.

The initial pdx is stored in frametable_base_pdx, so we can subtract the
result of pfn_to_pdx(start_address) from nr_pdxs; we know that we don't
need to cover any memory in the range 0-start in the frametable.

Remove the variable `nr_pages' because it is unused.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
CC: JBeulich@suse.com
(cherry picked from commit f51027be0688540aaab61513b06a8693a37e4c00)