[SCSI] libfcoe: add checking disable flag in FIP_FKA_ADV

When the D bit is set if the FKA_ADV_Period of the FIP Discovery
Advertisement, the ENode should not transmit period ENode FIP Keep Alive and
VN_Port FIP Keep Alive (FC-BB-5 Rev2, 7.8.3.13).

Note that fcf->flags is taken directly from the fip_header, I am claiming one
bit for the purpose of the FIP_FKA_Period D bit as FIP_FL_FK_ADV_B, and use
FIP_HEADER_FLAGS as bitmask for bits used in fip_header.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>