dmaengine: sf-pdma: pdma_desc memory leak fix

Commit b2cc5c465c2c ("dmaengine: sf-pdma: Add multithread support for a
DMA channel") changed sf_pdma_prep_dma_memcpy() to unconditionally
allocate a new sf_pdma_desc each time it is called.

The driver previously recycled descs, by checking the in_use flag, only
allocating additional descs if the existing one was in use. This logic
was removed in commit b2cc5c465c2c ("dmaengine: sf-pdma: Add multithread
support for a DMA channel"), but sf_pdma_free_desc() was not changed to
handle the new behaviour.

As a result, each time sf_pdma_prep_dma_memcpy() is called, the previous
descriptor is leaked, over time leading to memory starvation:

unreferenced object 0xffffffe008447300 (size 192):
comm "irq/39-mchp_dsc", pid 343, jiffies 4294906910 (age 981.200s)
hex dump (first 32 bytes):
00 00 00 ff 00 00 00 00 b8 c1 00 00 00 00 00 00 ................
00 00 70 08 10 00 00 00 00 00 00 c0 00 00 00 00 ..p.............
backtrace:
[<00000000064a04f4>] kmemleak_alloc+0x1e/0x28
[<00000000018927a7>] kmem_cache_alloc+0x11e/0x178
[<000000002aea8d16>] sf_pdma_prep_dma_memcpy+0x40/0x112

Add the missing kfree() to sf_pdma_free_desc(), and remove the redundant
in_use flag.

Fixes: b2cc5c465c2c ("dmaengine: sf-pdma: Add multithread support for a DMA channel")
Signed-off-by: Shravan Chippa <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/sf-pdma/sf-pdma.c

···
 	if (!desc)
 		return NULL;
 
-	desc->in_use = true;
 	desc->dirn = DMA_MEM_TO_MEM;
 	desc->async_tx = vchan_tx_prep(&chan->vchan, &desc->vdesc, flags);
 
···
 	struct sf_pdma_desc *desc;
 
 	desc = to_sf_pdma_desc(vdesc);
-	desc->in_use = false;
+	kfree(desc);
 }
 
 static void sf_pdma_donebh_tasklet(struct tasklet_struct *t)

drivers/dma/sf-pdma/sf-pdma.h

···
 	u64				src_addr;
 	struct virt_dma_desc		vdesc;
 	struct sf_pdma_chan		*chan;
-	bool				in_use;
 	enum dma_transfer_direction	dirn;
 	struct dma_async_tx_descriptor *async_tx;
 };