Fix nits in previous commit:

1. Remove initializer for badstack_sbuf_size; it gets set unconditionally.
2. Remove meaningless comment.
3. Group witness_count and its sysctl together.
4. Fix spacing in for statements (space after for and within condition).
5. Change *all* M_NOWAIT usages in witness_initialize() to M_WAITOK; not just those that were newly introduced -- the allocation is assumed to succeed for all allocations.
6. Avoid using uint8_t as the base type in sizeof() expressions; Use the variable name (w_rmatrix) as much as possible.

Pointed out by: jhb@ (thanks!)