sit: fix sparse warnings

This change fixes several sparse warnings about endianness problem. The wrong
nla_*() functions were used.
It also fix a sparse warning about a flag test (field i_flags). This field is
used in this file like a local flag only, so it is more an u16 (gre uses it as a
be16). This sparse warning was already there before the patch that add netlink
management, the code has just been moved.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>