xen/gntalloc: make grant counters unsigned

xen/gntalloc: make grant counters unsigned

The module limit and current allocation count cannot validly be
negative. Give both variables unsigned types so their representation
matches the u32 grant count supplied through the ioctl and negative
module parameter values are rejected by parameter parsing.

This also prepares the limit check for overflow-safe unsigned
arithmetic.

Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260626223805.43781-2-alhouseenyousef@gmail.com>