same problem on RHEL-7.x

+++ This bug was initially created as a clone of Bug #1401824 +++

Description of problem:

man exports
'''
       mountpoint=path

       mp     This option makes it possible to only export a directory if it has successfully been mounted.  If no path is given (e.g.  mountpoint or mp) then  the  export point  must also be a mount point.  If it isn’t then the export point is not exported.
'''

But We can always export a directory that's not mountpoint, even with -o mp option

[root@bkr-hv01-guest18 ~]# mountpoint /exportdir
/exportdir is not a mountpoint
[root@bkr-hv01-guest18 ~]# exportfs -if -o mp  '*:/exportdir'
[root@bkr-hv01-guest18 ~]# echo $?
0
[root@bkr-hv01-guest18 ~]# showmount -e localhost
Export list for localhost:
/exportdir *
[root@bkr-hv01-guest18 ~]# time mount localhost:/exportdir  /mnt/nfsmp


Version-Release number of selected component (if applicable):
RHEL-6 RHEL-7

How reproducible:
always

Steps to Reproduce:
1. mkdir /exportdir
2. echo "/exportdir localhost(mp)" >/etc/exports
3. service nfs restart
4. showmount -e localhost
         ^^^ export success
5. mount localhost:/exportdir /mnt/nfsmp
         ^^^ hangup here

Actual results:
/exportdir can be always exported successfully
  and cause client mount hangup

Expected results:
/exportdir should not be exported, if add mountpoint export option
  client mount should exit with an error message to user

Additional info: