cifs: Fix smb2_set_path_size()

If cifs_get_writable_path() finds a writable file, smb2_compound_op()
must use that file's FID and not the COMPOUND_FID.

Cc: stable@vger.kernel.org
Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>