convert cifs_ioctl_copychunk()

fdput() moved past mnt_drop_file_write(); harmless, if somewhat cringeworthy.
Reordering could be avoided either by adding an explicit scope or by making
mnt_drop_file_write() called via __cleanup.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>