cifs: fix filp leak in cifs_atomic_open()

If an error occurs after having called finish_open() then fput() needs to
be called on the already opened file.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Steve French <sfrench@samba.org>
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>