staging: vme_user: remove buf_unalloc helper

buf_unalloc is essentially a vme_free_consistent:
1) image[i].kern_buf is never NULL in buf_alloc call
2) kern_buf, pci_buf and size_buf get zeroed in vme_user_probe anyway

Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
Cc: Igor Alekseev <igor.alekseev@itep.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>