scsi: libfc: Fix passing zero to 'PTR_ERR' warning

drivers/scsi/libfc/fc_disc.c:304
 fc_disc_error() warn: passing zero to 'PTR_ERR'

fp may be NULL in fc_disc_error(), use PTR_ERR_OR_ZERO to handle this.

Link: https://lore.kernel.org/r/20200909135432.36772-1-yuehaibing@huawei.com


Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>