ip6/tunnel: Unify tclass printing

ip6/tunnel: Unify tclass printing

Use @s2 buffer to store string representation of
tclass and get rid of extra SPRINT_BUF(): no
need to preserve @s2 contents for later.

Use print_string(PRINT_ANY, ...) with prepared by
snprintf() string for both PRINT_JSON and PRINT_FP
cases.

While there use __u32 for flowinfo in link_gre6.c
and check for IFLA_GRE_FLOWINFO attribute presense.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>