ip6/tunnel: Unify flowlabel printing

ip6/tunnel: Unify flowlabel printing

Use @s2 buffer to store string representation of
flowlabel 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.

Omit flowlabel from output if no flowinfo attribute
is given and IP6_TNL_F_USE_ORIG_FLOWLABEL isn't set.

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