iptunnel: check SIT_ISATAP flag only for SIT tunnel

iptunnel: check SIT_ISATAP flag only for SIT tunnel

Without patch, I got:

	# ./ip/ip tunnel show
	ip_vti0: ioctl 89f4 failed: Invalid argument
	ip_vti0: ip/ip  remote any  local any  ttl inherit  nopmtudisc key 0

this is due to VTI_ISVTI has the same numeric value with SIT_ISATAP,
but only sit tunnel has SIOCGETPRL, therefore it should check for SIT
tunnel first.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Cong Wang <amwang@redhat.com>