lmb: Rename free_mem to available_mem

free_mem is a misnomer. We never update it with the free memory for
LMB. Instead, it describes all available memory and is checked against
used_mem to decide whether an area is free or not.

So let's rename this field to better match its usage.

Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Tested-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/test/lib/lmb.c b/test/lib/lmb.c
index 3c3e862..6e87027 100644
--- a/test/lib/lmb.c
+++ b/test/lib/lmb.c
@@ -65,7 +65,7 @@
 
 	ut_assertok(lmb_push(store));
 	lmb = lmb_get();
-	*mem_lstp = &lmb->free_mem;
+	*mem_lstp = &lmb->available_mem;
 	*used_lstp = &lmb->used_mem;
 
 	return 0;