um: Fix return value in ubd_init()

When kmalloc_array() fails to allocate memory, the ubd_init()
should return -ENOMEM instead of -1. So, fix it.

Fixes: f88f0bdf ("um: UBD Improvements")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>