netlink_smc_diag: implement SMC_DIAG_DMBINFO decoding

netlink_smc_diag: implement SMC_DIAG_DMBINFO decoding

The message has been added by Linux commit v4.19-rc1~140^2~507^2~1.

* linux/smc_diag.h: Include "gcc_compat.h".
(enum): Add SMC_DIAG_DMBINFO.
(struct smcd_diag_dmbinfo): New type definition.
* xlat/smc_diag_attrs.in (SMC_DIAG_DMBINFO): New constant.
* netlink_smc_diag.c (decode_smc_diag_dmbinfo): New function.
(smc_diag_msg_nla_decoders) [SMC_DIAG_DMBINFO]: New decoder, calls
decode_smc_diag_dmbinfo.
* tests/nlattr_smc_diag_msg.c: Add SMC_DIAG_DMBINFO check.