net: af_key: fix sadb_x_filter validation

When running xfrm_state_walk_init(), the xfrm_address_filter being used
is okay to have a splen/dplen that equals to sizeof(xfrm_address_t)<<3.
This commit replaces >= to > to make sure the boundary checking is
correct.

Fixes: 37bd2242 ("af_key: pfkey_dump needs parameter validation")
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>