NFSD: Document lack of f_pos_lock in nfsd_readdir()

Al Viro notes that normal system calls hold f_pos_lock when calling
->iterate_shared and ->llseek; however nfsd_readdir() does not take
that mutex when calling these methods.

It should be safe however because the struct file acquired by
nfsd_readdir() is not visible to other threads.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>