ip/tunnel: Minor cleanups

ip/tunnel: Minor cleanups

Few minor changes to reduce diffs between ip and ipv6 tunnel code:

  1) reduce intendation by one level when adding attributes in gre and
     gre6; reorder addattr*() calls to simplify diff

  2) reorder local variables definition; change their type (e.g. for
     IFLA_LINK) to match ones returned by rta_getattr_*()

  3) move "mode" parameter parsing in link_iptnl.c to the similar
     position as in link_ip6tnl.c

  4) handle "tc" as shortcut for "tclass"/"tos" in link_iptnl.c

  5) add whitespace where required

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>