NFSD: Protect against send buffer overflow in NFSv2 READDIR

Restore the previous limit on the @count argument to prevent a
buffer overflow attack.

Fixes: 53b1119a

 ("NFSD: Fix READDIR buffer overflow")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>