vfs: mnt_drop_write_file()

new helper (wrapper around mnt_drop_write()) to be used in pair with
mnt_want_write_file().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/inode.c b/fs/inode.c
index ee4e66b..4fda5ee 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1508,7 +1508,7 @@
 	if (sync_it & S_MTIME)
 		inode->i_mtime = now;
 	mark_inode_dirty_sync(inode);
-	mnt_drop_write(file->f_path.mnt);
+	mnt_drop_write_file(file);
 }
 EXPORT_SYMBOL(file_update_time);