The futex and the compat syscall entry points do pretty much the same
except for the timespec data types and the corresponding copy from
user function.

Split out the rest into inline functions and share the functionality.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210422194705.244476369@linutronix.de