Fix gcc warnings

Add WRAPPED_CTASSERT macro by annotating CTASSERTs with __unused
to deal with -Wunused-local-typedefs warnings from gcc 4.8+.
All other compilers (clang, etc) use CTASSERT as-is. A more generic
solution for this issue will be proposed after ^/stable/11 is forked.

Consolidate all CTASSERTs under one block instead of inlining them in
functions.

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7119
MFC after: 1 week
Reported by: Jenkins
Reviewed by: grehan (maintainer)
Sponsored by: EMC / Isilon Storage Division