ss: Make struct tcpstat fields 'timer' and 'timeout' unsigned

ss: Make struct tcpstat fields 'timer' and 'timeout' unsigned

Both 'timer' and 'timeout' variables of struct tcpstat are either
scanned as unsigned values from /proc/net/tcp{,6} or copied from
'idiag_timer' and 'idiag_expries' fields of struct inet_diag_msg, which
itself are unsigned. Therefore they may be unsigned as well, which
eliminates the need to check for negative values.

Signed-off-by: Phil Sutter <phil@nwl.cc>