Use buf + strategy rather than bypassing geom_vfs layer

The reference counting that's done in the geom_vfs layer to prevent
delivery of requests to defunct devices only works if all requests go
through that layer. UFS was bypassing that layer for BIO_DELETE requests,
sending them to the geom_consumer directly with g_io_request. Allocate
a buf, fill it in, and call strategy on it instead.

Submitted by: Chuck Silvers
Reviewed by: scottl, imp, kirk
Sponsored by: Netflix
Differential: https://reviews.freebsd.org/D15456