ip/tunnel: Use print_0xhex() instead of print_string()

ip/tunnel: Use print_0xhex() instead of print_string()

No need for custom SPRINT_BUF() and snprintf() 0x%x
value to this buffer: we can use print_0xhex() instead
of print_string().

In link_iptnl.c use s2 instead of s1 buffer and remove
s1.

While there adjust fwmark option print order in iptnl
and ip6tnl to get it match each other.

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