input: calloc minimization for xi2mask_new

input: calloc minimization for xi2mask_new

There's no reason to do this as (nmasks + 2) callocs, and it's a
surprisingly hot path.  Turns out you hit this ~once per passive grab,
and you do a few bajillion passive grab changes every time you enter or
leave the overview in gnome-shell.  According to a callgrind of Xorg
with gnome-shell-perf-tool run against it:

Ir before: 721437275
Ir after:  454227086

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Jasper St. Pierre <jstpierre@mecheye.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>