NFSD: prevent underflow in nfssvc_decode_writeargs()

commit 184416d4b98509fb4c3d8fc3d6dc1437896cc159 upstream.

Smatch complains:

	fs/nfsd/nfsxdr.c:341 nfssvc_decode_writeargs()
	warn: no lower bound on 'args->len'

Change the type to unsigned to prevent this issue.

Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>