nfsd: fix dentry refcounting on create

b44061d0b9 introduced a dentry ref counting bug.  Previously we were
grabbing one ref to dchild in nfsd_create(), but with the creation of
nfsd_create_locked() we have a ref for dchild from the lookup in
nfsd_create(), and then another ref in nfsd_create_locked().  The ref
from the lookup in nfsd_create() is never dropped and results in
dentries still in use at unmount.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Fixes: b44061d0b9 "nfsd: reorganize nfsd_create"
Reported-by: kernel test robot <xiaolong.ye@intel.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/vfs.c

@@ -1252,10 +1252,13 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	if (IS_ERR(dchild))
 		return nfserrno(host_err);
 	err = fh_compose(resfhp, fhp->fh_export, dchild, fhp);
-	if (err) {
+	/*
+	 * We unconditionally drop our ref to dchild as fh_compose will have
+	 * already grabbed its own ref for it.
+	 */
 	dput(dchild);
+	if (err)
 		return err;
-	}
 	return nfsd_create_locked(rqstp, fhp, fname, flen, iap, type,
 					rdev, resfhp);
 }