scsi: lpfc: Terminate string in lpfc_debugfs_nvmeio_trc_write()

The "mybuf" string comes from the user, so we need to ensure that it is NUL
terminated.

Link: https://lore.kernel.org/r/20211214070527.GA27934@kili
Fixes: bd2cdd5e

 ("scsi: lpfc: NVME Initiator: Add debugfs support")
Reviewed-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>