Use VLAN_TAG_VALUE() not only to read a dot1q tag

Use VLAN_TAG_VALUE() not only to read a dot1q tag
value from an m_tag, but also to set it.  This reduces
complex code duplication and improves its readability.

Alas, we shouldn't rename the macro to VLAN_TAG_LVALUE()
globally because that would cause pain for kernel module
port maintainers and vendors using FreeBSD as their codebase.
Added a clarifying comment instead.

Discussed with:	ru, glebius
X-MFC-After:	6.0-RELEASE (MFC is good just to reduce the diff)