Fix a potential use-after-free in zfs_setsecattr()

In general, VOPs must not load the "z_log" field until having called
zfs_enter_verify_zp().

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #15752