net/xen-netback: off by one in BUG_ON() condition

net/xen-netback: off by one in BUG_ON() condition

The > should be >=.  I also added spaces around the '-' operations so
the code is a little more consistent and matches the condition better.

Fixes: f53c3fe8dad7 ('xen-netback: Introduce TX grant mapping')
Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 50c2e4dd6749725338621fff456b26d3a592259f)
Signed-off-by: Annie Li <annie.li@oracle.com>