iplink: add gso and gro max_size attributes for ipv4

iplink: add gso and gro max_size attributes for ipv4

This patch adds two attributes gso/gro_ipv4_max_size in iplink for the
user space support of the BIG TCP for IPv4:

  https://lore.kernel.org/netdev/de811bf3-e2d8-f727-72bc-c8a754a9d929@tessares.net/T/

Note that after this kernel patchset, "gso/gro_max_size" are used for IPv6
packets while "gso/gro_ipv4_max_size" are for IPv4 patckets. To not break
these old applications using "gso/gro_ipv4_max_size" for IPv4 GSO packets,
the new size will also be set on "gso/gro_ipv4_max_size" in kernel when
"gso/gro_max_size" changes to a value <= 65536.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David Ahern <dsahern@kernel.org>