vt_core.c: Use do/while to highlight missed semi-colon errors

Also, fix some nearby #define whitespace while here.

(Style cleanup for r285794.)

Suggested by: jmg

Differential Revision: https://reviews.freebsd.org/D3154
Approved by: markj (mentor)
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division