shsec: Allocate data blocks only for BIO_READ/WRITE requests

In particular, there is no need to allocate a data block when passing
BIO_FLUSH requests to child providers, and g_io_request() asserts that
bp->bio_data == NULL for such requests.

PR: 255131
Reported and tested by: nvass@gmx.com
Sponsored by: The FreeBSD Foundation

(cherry picked from commit a2dfffb98917a57bfacb155b9d7d423c3e8ff792)