dma-buf: don't open-code atomic_long_read()

... not to mention that even atomic_long_read() is too low-level here -
there's file_count().
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/dma-buf/dma-buf.c

@@ -799,7 +799,7 @@ static int dma_buf_describe(struct seq_file *s)
 		seq_printf(s, "%08zu\t%08x\t%08x\t%08ld\t%s\n",
 				buf_obj->size,
 				buf_obj->file->f_flags, buf_obj->file->f_mode,
-				(long)(buf_obj->file->f_count.counter),
+				file_count(buf_obj->file),
 				buf_obj->exp_name);
 
 		seq_puts(s, "\tAttached Devices:\n");