freebsd: changes necessary to coexist with dtrace in tree

Fix header conflicts when building zfs with openzfs as a vendor import.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10497