pfkey: fix SADB_X_EXT_FILTER length check

This patch fixes commit d3623099 ("ipsec: add support of limited SA dump").

sadb_ext_min_len array should be updated with the new type (SADB_X_EXT_FILTER).

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>