lift sb_start_write() out of ->write()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index 4809922..317f9ee 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -962,12 +962,14 @@
 			}
 
 			data = kmap(page);
+			file_start_write(file);
 			old_fs = get_fs();
 			set_fs(KERNEL_DS);
 			ret = file->f_op->write(
 				file, (const void __user *) data, len, &pos);
 			set_fs(old_fs);
 			kunmap(page);
+			file_end_write(file);
 			if (ret != len)
 				ret = -EIO;
 		}