cred: Do not default to init_cred in prepare_kernel_cred()

A common exploit pattern for ROP attacks is to abuse prepare_kernel_cred()
in order to construct escalated privileges[1]. Instead of providing a
short-hand argument (NULL) to the "daemon" argument to indicate using
init_cred as the base cred, require that "daemon" is always set to
an actual task. Replace all existing callers that were passing NULL
with &init_task.

Future attacks will need to have sufficiently powerful read/write
primitives to have found an appropriately privileged task and written it
to the ROP stack as an argument to succeed, which is similarly difficult
to the prior effort needed to escalate privileges before struct cred
existed: locate the current cred and overwrite the uid member.

This has the added benefit of meaning that prepare_kernel_cred() can no
longer exceed the privileges of the init task, which may have changed from
the original init_cred (e.g. dropping capabilities from the bounding set).

[1] https://google.com/se...