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
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation