[TCP]: Remove unneeded implicit type cast when calling tcp_minshall_update()

The tcp_minshall_update() function is called in exactly one place, and is
passed an unsigned integer for the mss_len argument.  Make the sign of the
argument match the sign of the passed variable in order to eliminate an
unneeded implicit type cast and a mixed sign comparison in
tcp_minshall_update().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>