vtblk: Make vtblk_request_execute return void.

The error, if any, now gets stashed in the request structure. (Step 1
of reworking this driver to use busdma.)

No functional change intended.

Reviewed by: bryanv, imp
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D36664