sfxge(4): get max supported value for action MARK

The mark value for MATCH_ACTION_MARK has a maximum value.
Requesting a value larger than the maximum will cause the
filter insertion to fail with EINVAL. This patch allows the
driver to check the value at the filter validation.

Submitted by: Roman Zhukov <roman.zhukov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18240