ip-xfrm: Add support for OUTPUT_MARK

ip-xfrm: Add support for OUTPUT_MARK

This patch adds support for OUTPUT_MARK in xfrm state to exercise the
functionality added by kernel commit 077fbac405bf
("net: xfrm: support setting an output mark.").

Sample output-

(with mark and output-mark)
src 192.168.1.1 dst 192.168.1.2
        proto esp spi 0x00004321 reqid 0 mode tunnel
        replay-window 0 flag af-unspec
        mark 0x10000/0x3ffff output-mark 0x20000
        auth-trunc xcbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b211 96
        enc cbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b233
        anti-replay context: seq 0x0, oseq 0x0, bitmap 0x00000000

(with mark only)
src 192.168.1.1 dst 192.168.1.2
        proto esp spi 0x00004321 reqid 0 mode tunnel
        replay-window 0 flag af-unspec
        mark 0x10000/0x3ffff
        auth-trunc xcbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b211 96
        enc cbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b233
        anti-replay context: seq 0x0, oseq 0x0, bitmap 0x00000000

(with output-mark only)
src 192.168.1.1 dst 192.168.1.2
        proto esp spi 0x00004321 reqid 0 mode tunnel
        replay-window 0 flag af-unspec
        output-mark 0x20000
        auth-trunc xcbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b211 96
        enc cbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b233
        anti-replay context: seq 0x0, oseq 0x0, bitmap 0x00000000

(no mark and output-mark)
src 192.168.1.1 dst 192.168.1.2
        proto esp spi 0x00004321 reqid 0 mode tunnel
        replay-window 0 flag af-unspec
        auth-trunc xcbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b211 96
        enc cbc(aes) 0x3ed0af408cf5dcbf5d5d9a5fa806b233
        anti-replay context: seq 0x0, oseq 0x0, bitmap 0x00000000

v1->v2: Moved the XFRMA_OUTPUT_MARK print after XFRMA_MARK in
xfrm_xfrma_print() as mentioned by Lorenzo

v2->v3: Fix one help formatting error as mentioned by Lorenzo.
Keep mark and output-mark on the same line and add man page info as
mentioned by David.

Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David Ahern <dsahern@gmail.com>