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/hfsplus/ioctl.c b/fs/hfsplus/ioctl.c
index 31d3fe5..f66c765 100644
--- a/fs/hfsplus/ioctl.c
+++ b/fs/hfsplus/ioctl.c
@@ -94,7 +94,7 @@
 out_unlock_inode:
 	mutex_unlock(&inode->i_mutex);
 out_drop_write:
-	mnt_drop_write(file->f_path.mnt);
+	mnt_drop_write_file(file);
 out:
 	return err;
 }