zvol_os.c: Increase optimal IO size

Since zvol read and write can process up to (DMU_MAX_ACCESS / 2) bytes
in a single operation, the current optimal I/O size is too low. SCST
directly reports this value as the optimal transfer length for the
target SCSI device. Increasing it from the previous volblocksize results
in performance improvement for large block parallel I/O workloads.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #16750