vtblk: Fix dumping

Now that vtblk uses busdma, it keeps important information inside its
request structures. The functions used for kernel dumps synthesize
their own request structures rather than using structures initialized
with the necessary bits for busdma.

Add busdma-bypass paths. Since dumping writes contiguous blocks of
physical memory, vtblk doesn't need busdma in that case.

Reported by: glebius
Tested by: glebius
Differential Revision: https://reviews.freebsd.org/D37243