nvmet-fcloop: suppress a compiler warning

Building with W=1 enables the compiler warning -Wimplicit-fallthrough=3. That
option does not recognize the fall-through comment in the fcloop driver. Add
a fall-through comment that is recognized for -Wimplicit-fallthrough=3. This
patch avoids that the compiler reports the following warning when building
with W=1:

drivers/nvme/target/fcloop.c:647:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (op == NVMET_FCOP_READDATA)
      ^
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/fcloop.c

@@ -648,6 +648,7 @@ fcloop_fcp_op(struct nvmet_fc_target_port *tgtport,
 			break;
 
 		/* Fall-Thru to RSP handling */
+		/* FALLTHRU */
 
 	case NVMET_FCOP_RSP:
 		if (fcpreq) {