posix-cpu-timers: Provide task validation functions

The code contains three slightly different copies of validating whether a
given clock resolves to a valid task and whether the current caller has
permissions to access it.

Create central functions. Replace check_clock() as a first step and rename
it to something sensible.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190821192919.326097175@linutronix.de