coresight: tmc-etr: Check if page is valid before dma_map_page()

alloc_pages_node() return should be checked before calling
dma_map_page() to make sure that valid page is mapped or
else it can lead to aborts as below:

 Unable to handle kernel paging request at virtual address ffffffc008000000
 Mem abort info:
 <snip>...
 pc : __dma_inv_area+0x40/0x58
 lr : dma_direct_map_page+0xd8/0x1c8

 Call trace:
  __dma_inv_area
  tmc_pages_alloc
  tmc_alloc_data_pages
  tmc_alloc_sg_table
  tmc_init_etr_sg_table
  tmc_alloc_etr_buf
  tmc_enable_etr_sink_sysfs
  tmc_enable_etr_sink
  coresight_enable_path
  coresight_enable
  enable_source_store
  dev_attr_store
  sysfs_kf_write

Fixes: 99443ea1 ("coresight: Add generic TMC sg table framework")
Cc: stable@vger.kernel.org
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20201127175256.1092685-13-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-tmc-etr.c

@@ -217,6 +217,8 @@ static int tmc_pages_alloc(struct tmc_pages *tmc_pages,
 		} else {
 			page = alloc_pages_node(node,
 						GFP_KERNEL | __GFP_ZERO, 0);
+			if (!page)
+				goto err;
 		}
 		paddr = dma_map_page(real_dev, page, 0, PAGE_SIZE, dir);
 		if (dma_mapping_error(real_dev, paddr))