On the first allocation of a task the I/O bitmap needs to be
allocated. After the allocation it is installed as an empty bitmap and
immediately afterwards updated.

Avoid that and just do the initial updates (store bitmap pointer, set TIF
flag and make TSS limit valid) in the update path unconditionally. If the
bitmap was already active this is redundant but harmless.

Preparatory change for later optimizations in the context switch code.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>