prison_check_nfsd: Add check for enforce_statfs != 0

Since mountd(8) will not be able to do exports
when running in a vnet prison if enforce_statfs is
set to 0, add a check for this to prison_check_nfsd().

Reviewed by:	jamie, markj
MFC after:	2 months
Differential Revision:	https://reviews.freebsd.org/D38189

sys/kern/kern_jail.c

@@ -3491,6 +3491,8 @@ prison_check(struct ucred *cred1, struct ucred *cred2)
  * - The root directory (pr_root) of the prison must be
  *   a file system mount point, so the mountd can hang
  *   export information on it.
+ * - The prison's enforce_statfs cannot be 0, so that
+ *   mountd(8) can do exports.
  */
 bool
 prison_check_nfsd(struct ucred *cred)
@@ -3502,6 +3504,8 @@ prison_check_nfsd(struct ucred *cred)
 		return (false);
 	if ((cred->cr_prison->pr_root->v_vflag & VV_ROOT) == 0)
 		return (false);
+	if (cred->cr_prison->pr_enforce_statfs == 0)
+		return (false);
 	return (true);
 }