FreeBSD: Optimize large kstat outputs

• Use sbuf_new_for_sysctl() to reduce double-buffering on sysctl

output.

• Use much faster sbuf_cat() instead of sbuf_printf("%s").

Together it reduces sysctl kstat.zfs.misc.dbufs time from minutes
to seconds, making dbufstat almost usable.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15495