nfs: fix handling of invalid mount options in nfs_remount

nfs_parse_mount_options returns 0 on error, not -errno.

Reported-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>