knfsd: Fixed problem with NFS exporting directories which are mounted on.

Recent changes in NFSd cause a directory which is mounted-on
to not appear properly when the filesystem containing it is exported.

*exp_get* now returns -ENOENT rather than NULL and when
  commit 5d3dbbea
removed the NULL checks, it didn't add a check for -ENOENT.
Signed-off-by: NNeil Brown <neilb@suse.de>
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/nfsd/vfs.c

@@ -115,7 +115,8 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 
 	exp2 = rqst_exp_get_by_name(rqstp, mnt, mounts);
 	if (IS_ERR(exp2)) {
-		err = PTR_ERR(exp2);
+		if (PTR_ERR(exp2) != -ENOENT)
+			err = PTR_ERR(exp2);
 		dput(mounts);
 		mntput(mnt);
 		goto out;