ip xfrm: support printing XFRMA_SET_MARK_MASK attribute in states

ip xfrm: support printing XFRMA_SET_MARK_MASK attribute in states

The XFRMA_SET_MARK_MASK attribute is set in states (4.19+).
It is the mask of XFRMA_SET_MARK(a.k.a. XFRMA_OUTPUT_MARK in 4.18)

sample output: note the output-mark mask
ip xfrm state
	src 192.1.2.23 dst 192.1.3.33
	proto esp spi 0xSPISPI reqid REQID mode tunnel
	replay-window 32 flag af-unspec
	output-mark 0x3/0xffffff
	aead rfc4106(gcm(aes)) 0xENCAUTHKEY 128
	if_id 0x1

Signed-off-by: Antony Antony <antony@phenome.org>
Signed-off-by: David Ahern <dsahern@gmail.com>