ip/tunnel: Use tnl_parse_key() to parse tunnel key

ip/tunnel: Use tnl_parse_key() to parse tunnel key

It is added with
commit a7ed1520ee96 ("ip/tunnel: introduce tnl_parse_key()")
to avoid code duplication in ip6?tunnel.c.

Reuse it for gre/gre6 and vti/vti6 tunnel rtnl
configuration interface with the same purpose
it is used in tunnel ioctl interface in ip6?tunnel.c.

While there change type of key variables from
unsigned integer to __be32 to reflect nature of the
value they store and place error message in
tnl_parse_key() on a single line to make single
call to fprintf().

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