cbus: simplify cbus_call interface

All of the current cbus_call() invocations pass TIMEOUT_INFINITY as a
timeout and don't provide any free_cb.

Since commit bd6fb06a ("core: allow
spurious wakeups in cbus_call") cbus_call() with TIMEOUT_INFINITY can't
be interrupted at all, so  there's no point to pass free_cb for it.

Let's simplify cbus_call() usage by removing the last two arguments,
free_cb and timeout.

These arguments will be present in a new function, cbus_call_timeout.

Simplify all current cbus_call() usages which have TIMEOUT_INFINITY.
Remove free_cb, where present, and stop allocating call messages
dynamically, since static allocation works just fine now.

NO_DOC=refactoring
NO_CHANGELOG=refactoring