esp: Simplify the calculation of variables

Fix the following coccicheck warnings:

./net/ipv6/esp6.c:791:16-18: WARNING !A || A && B is equivalent
to !A || B.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>