smbios: Add length sanity checking

D28743 was commited, reverted and then f689cb23b2782 landed before it
was recommitted. However, D28743 included an extra length check. Redo
that functionality so we check both the number of entries as well as the
length checks for wacky data.

Sponsored by: Netflix
Reviewed by: gallatin
Differential Revision: https://reviews.freebsd.org/D45763