Description of Problem:
exportfs -u fails to unexport a previously exported filesystem

Version-Release number of selected component (if applicable):
0.3.1-12

How Reproducible:
100%

Steps to Reproduce:
1. add "/mnt/cdrom *" to /etc/exports
2. exportfs -a
3. on client, "mount server:/mnt/cdrom /mnt/s-cdrom"
4. on client, "du /mnt/s-cdrom"
5. on client, "umount /mnt/s-cdrom"
6. on server, "exportfs -u client:/home/test"
7. on server, "umount /mnt/cdrom"

Actual Results:
step 7 fails because step 6 actually failed to remove the entry from the
kernel's table, so the kernel still has (presumably the root inode) still
open, preventing the umount from succeeding

Expected Results:
step 7 should succeed, because step 6 should tell the kernel to let go

Additional Information:
If (after step 7) you comment out the line added in step 1, then execute
"exportfs -r", re-executing step 7 works as expected, so there is something
 done correctly by the "-r" path that is not being done in the "-u" path.