[SCSI] libfc: add definition for task attribute mask

The FCP command header definition should define a mask for
the task attribute field.  This adds that #define.

Signed-off-by: Joe Eykholt <jeykholt@cisco.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>