Implement secpolicy_vnode_setid_retain()

Don't unconditionally return 0 (i.e. retain SUID/SGID).
Test CAP_FSETID capability.

https://github.com/pjd/pjdfstest/blob/master/tests/chmod/12.t
which expects SUID/SGID to be dropped on write(2) by non-owner fails
without this. Most filesystems make this decision within VFS by using
a generic file write for fops.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #9035
Closes #9043

module/zfs/policy.c

@@ -209,7 +209,7 @@ secpolicy_vnode_setdac(const cred_t *cr, uid_t owner)
 int
 secpolicy_vnode_setid_retain(const cred_t *cr, boolean_t issuidroot)
 {
-	return (0);
+	return (priv_policy_user(cr, CAP_FSETID, B_FALSE, EPERM));
 }
 
 /*