dma-buf: Fix static checker warning

Here is the warning message:

	drivers/dma-buf/dma-buf.c:917 dma_buf_map_attachment()
	error: 'sg_table' dereferencing possible ERR_PTR()

Fix by adding error checking before dereferencing the pointer.

Fixes: ac80cd17

 ("dma-buf: Clarify that dma-buf sg lists are page aligned")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jianxin Xiong <jianxin.xiong@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/398485/