IB/mthca: Use uninitialized_var() for f0

Commit 9db48926 ("drivers/infiniband/hw/mthca/mthca_qp: kill uninit'd
var warning") added "= 0" to the declarations of f0 to shut up gcc
warnings.  However, there's no point in making the code bigger by
initializing f0 to a random value just to get rid of a warning;
setting f0 to 0 is no safer than just using uninitialized_var(), which
documents the situation better and gives smaller code too.  For example, 
on x86_64:

add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-16 (-16)
function                                     old     new   delta
mthca_tavor_post_send                       1352    1344      -8
mthca_arbel_post_send                       1489    1481      -8
Signed-off-by: NRoland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mthca/mthca_qp.c

@@ -1591,7 +1591,13 @@ int mthca_tavor_post_send(struct ib_qp *ibqp, struct ib_send_wr *wr,
 	int i;
 	int size;
 	int size0 = 0;
-	u32 f0 = 0;
+	/*
+	 * f0 is only used if nreq != 0, and f0 will be initialized
+	 * the first time through the main loop, since size0 == 0 the
+	 * first time through.  So nreq cannot become non-zero without
+	 * initializing f0, and f0 is in fact never used uninitialized.
+	 */
+	u32 uninitialized_var(f0);
 	int ind;
 	u8 op0 = 0;
 
@@ -1946,7 +1952,13 @@ int mthca_arbel_post_send(struct ib_qp *ibqp, struct ib_send_wr *wr,
 	int i;
 	int size;
 	int size0 = 0;
-	u32 f0 = 0;
+	/*
+	 * f0 is only used if nreq != 0, and f0 will be initialized
+	 * the first time through the main loop, since size0 == 0 the
+	 * first time through.  So nreq cannot become non-zero without
+	 * initializing f0, and f0 is in fact never used uninitialized.
+	 */
+	u32 uninitialized_var(f0);
 	int ind;
 	u8 op0 = 0;