Backport MHSIZE/MPKTHSIZE equivalents from head

These macros are equivalent to the ones on head, except they are only exposed
when _KERNEL is defined, i.e. to kernel code, whereas the code on head is exposed
to userland as well

This is for improved forwards compatibility with mbuf(9) macros in head@r277203+,
and is required for a clean MFC of r279393

This is a direct commit to stable/10

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D2126
Reviewed by: glebius, rwatson
Sponsored by: EMC / Isilon Storage Division