bridge: avoid snprint truncation on time

bridge: avoid snprint truncation on time

This fixes new gcc warning about possible string overflow.

mdb.c: In function ‘__print_router_port_stats’:
mdb.c:61:11: warning: ‘%.2i’ directive output may be truncated
 writing between 2 and 7 bytes into a region of size
 between 0 and 4 [-Wformat-truncation=]
      "%4i.%.2i", (int)tv.tv_sec,
           ^~~~
Note: already fixed in iproute2-next.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>