[SCSI] fcoe: Prevent creation of an NPIV port with duplicate WWPN

This patch adds a validation step before allowing creation of a new NPIV port.
It checks whether the WWPN passed for the new NPIV port to be created is unique
for the given physical port.

Signed-off-by: Neerav Parikh <Neerav.Parikh@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <jbottomley@parallels.com>