[SCSI] libfc: discovery gpn_ft parse bug

In fc_disc_gpn_ft_parse(), after fc_disc_done() is called, the
disc state is changed by setting buf_len = 0.  This is wrong
since the discovery may have restarted.   Instead, return
after calling fc_disc_done.

Also, return an error on memory allocation failure.

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>