Consolidate zfs_holey and zfs_access

The zfs_holey() and zfs_access() functions can be made common
to both FreeBSD and Linux.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #11125