UBIFS: avoid unnecessary calculations

Do not calculate min_idx_lebs, because it is available in
c->min_idx_lebs

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/budget.c b/fs/ubifs/budget.c
index 0bcb803..44cff80 100644
--- a/fs/ubifs/budget.c
+++ b/fs/ubifs/budget.c
@@ -763,7 +763,8 @@
 	long long available, outstanding, free;
 
 	spin_lock(&c->space_lock);
-	min_idx_lebs = ubifs_calc_min_idx_lebs(c);
+	min_idx_lebs = c->min_idx_lebs;
+	ubifs_assert(min_idx_lebs == ubifs_calc_min_idx_lebs(c));
 	outstanding = c->budg_data_growth + c->budg_dd_growth;
 
 	/*