block: Rename disk_op_s->drive_gf to drive_fl

Now that the drive_s struct does not need to be in the f-segment,
rename references to drive_gf in the generic drive code to drive_fl.

This is just variable renames - no code changes.

Tested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/hw/nvme.c b/src/hw/nvme.c
index 556a9a4..93c25f5 100644
--- a/src/hw/nvme.c
+++ b/src/hw/nvme.c
@@ -601,7 +601,7 @@
     if (!CONFIG_NVME)
         return DISK_RET_SUCCESS;
 
-    struct nvme_namespace *ns = container_of(op->drive_gf, struct nvme_namespace,
+    struct nvme_namespace *ns = container_of(op->drive_fl, struct nvme_namespace,
                                              drive);
 
     switch (op->command) {