vtblk: secondary fix for dumping

The code paths while dumping do not got through busdma. As such,
safeguard against calling bus_dmamap_sync() with a NULL map. The x86
implementation tolerates this but others do not, resulting in a
NULL dereference panic when dumping to a vtblk device on arm64, riscv,
etc.

Fixes: 782105f7c898 ("vtblk: Use busdma")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37990