[XFRM] AUDIT: Fix flowlabel text format ambibuity.

Flowlabel text format was not correct and thus ambiguous.
For example, 0x00123 or 0x01203 are formatted as 0x123.
This is not what audit tools want.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>