lib/genalloc.c: add check gen_pool_dma_alloc() if dma pointer is not NULL

In the gen_pool_dma_alloc() the dma pointer can be NULL and while
assigning gen_pool_virt_to_phys(pool, vaddr) to dma caused the following
crash on da850 evm:

   Unable to handle kernel NULL pointer dereference at virtual address 00000000
   Internal error: Oops: 805 [#1] PREEMPT ARM
   Modules linked in:
   CPU: 0 PID: 1 Comm: swapper Tainted: G        W    3.13.0-rc1-00001-g0609e45-dirty #5
   task: c4830000 ti: c4832000 task.ti: c4832000
   PC is at gen_pool_dma_alloc+0x30/0x3c
   LR is at gen_pool_virt_to_phys+0x74/0x80
   Process swapper, call trace:
     gen_pool_dma_alloc+0x30/0x3c
     davinci_pm_probe+0x40/0xa8
     platform_drv_probe+0x1c/0x4c
     driver_probe_device+0x98/0x22c
     __driver_attach+0x8c/0x90
     bus_for_each_dev+0x6c/0x8c
     bus_add_driver+0x124/0x1d4
     driver_register+0x78/0xf8
     platform_driver_probe+0x20/0xa4
     davinci_init_late+0xc/0x14
     init_machine_late+0x1c/0x28
     do_one_initcall+0x34/0x15c
     kernel_init_freeable+0xe4/0x1ac
     kernel_init+0x8/0xec

This patch fixes the above.

[akpm@linux-foundation.org: update kerneldoc]
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Nicolin Chen <b42378@freescale.com>
Cc: Joe Perches <joe@perches.com>
Cc: Sachin Kamat <sachin.kamat@linaro.org>
Cc: <stable@vger.kernel.org>	[3.13.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/genalloc.c

@@ -316,7 +316,7 @@ EXPORT_SYMBOL(gen_pool_alloc);
  * gen_pool_dma_alloc - allocate special memory from the pool for DMA usage
  * @pool: pool to allocate from
  * @size: number of bytes to allocate from the pool
- * @dma: dma-view physical address
+ * @dma: dma-view physical address return value.  Use NULL if unneeded.
  *
  * Allocate the requested number of bytes from the specified pool.
  * Uses the pool allocation function (with first-fit algorithm by default).
@@ -334,6 +334,7 @@ void *gen_pool_dma_alloc(struct gen_pool *pool, size_t size, dma_addr_t *dma)
 	if (!vaddr)
 		return NULL;
 
+	if (dma)
 		*dma = gen_pool_virt_to_phys(pool, vaddr);
 
 	return (void *)vaddr;