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.

[[e-mail]: update kerneldoc]
Signed-off-by: Lad, Prabhakar <[e-mail]>
Cc: Philipp Zabel <[e-mail]>
Cc: Nicolin Chen <[e-mail]>
Cc: Joe Perches <[e-mail]>
Cc: Sachin Kamat <[e-mail]>
Cc: <[e-mail]> [3.13.x]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/genalloc.c

···
  * 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).
···
 	if (!vaddr)
 		return NULL;
 
-	*dma = gen_pool_virt_to_phys(pool, vaddr);
+	if (dma)
+		*dma = gen_pool_virt_to_phys(pool, vaddr);
 
 	return (void *)vaddr;
 }