Pack dmu_buf_impl_t by 16 bytes

On 64bit FreeBSD this reduces one from 296 to 280 bytes. On small
block workloads dbufs may consume gigabytes of ARC, and this saves
5% of it.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16684