xen: fix buffer over-read in bitmap_to_xenctl_bitmap()

xen: fix buffer over-read in bitmap_to_xenctl_bitmap()

There's an off-by-one when calculating the last byte in the input array to
bitmap_to_xenctl_bitmap(), which leads to bitmaps with sizes multiple of 8
to over-read and incorrectly use a byte past the end of the array.

Fixes: 288c4641c80d ('xen: simplify bitmap_to_xenctl_bitmap for little endian')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>