Enable PF_FSTRANS for ioctl secpolicy callbacks

At the very least, the zfs_secpolicy_write_perms ioctl security policy
callback, which calls dsl_dataset_hold(), can require freeing memory and,
therefore, re-enter ZFS.  This patch enables PF_FSTRANS for all of the
security policy callbacks similarly to the manner in which it's enabled
for the actual ioctl callback.

May-fix: #4554

module/zfs/zfs_ioctl.c

@@ -5831,8 +5831,11 @@ zfsdev_ioctl(struct file *filp, unsigned cmd, unsigned long arg)
 	}
 
 
-	if (error == 0 && !(flag & FKIOCTL))
+	if (error == 0 && !(flag & FKIOCTL)) {
+		cookie = spl_fstrans_mark();
 		error = vec->zvec_secpolicy(zc, innvl, CRED());
+		spl_fstrans_unmark(cookie);
+	}
 
 	if (error != 0)
 		goto out;