ethernet: Move the assertion of ether header sizes back into ethernet.h

There're lots of consumers, Ethernet drivers, libraries and applications.
It is more promising to assert the sizes in every compilation units
rather than in if_ethersubr.c only.

Those assertions were in the header file but were moved to if_ethersubr.c
due to possible conflict of the implementation of CTASSERT [1]. Now that
the default C standard is now gnu17 [2] [3] which supports _Static_assert
natively, use _Static_assert instead of CTASSERT to avoid possible
conflicts.

While here, add an extra assertion for the size of struct ether_vlan_header.

[1] d54d93ac7f0f Move CTASSERT of ether header sizes out of the header file and into ...
[2] ca4eddea97c5 src: Use gnu17 as the default C standard for userland instead of gnu99
[3] 3a98d5701c7f sys: Use gnu17 as the default C standard for the kernel

PR: 287761 (exp-run)
Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D50947