Fix -Wattribute-warning in zfs_log_xvattr()

Restructure the code in zfs_log_xvattr() to use a lr_attr_end
structure when accessing lr_attr_t elements located after the
variable sized array. This makes the code more understandable
and resolves the accessing beyond the end of the field warnings.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13528
Closes #13575